L.* /J

ANALYSIS OF NORTHEAST PACIFIC HURRICANE TICO AND ITS ASSOCIATED HEAVY RAIN EVENT OVER THE SOUTH PLAINS OF AND

by JOEL LEE TUMBIOLO, B.S. A THESIS IN ATMOSPHERIC SCIENCE

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE

Approved

Accepted

May, 1989 ACKNOWLEDGMENTS

I wish to extend my sincere thanks to Dr. Richard E. Peterson for his encouragements, assistance, and understanding during the preparation of this thesis. I would also like to thank Dr. Colleen A. Leary and Dr. Chia Bo Chang for their assistance and advice during the course of this work. A special thanks goes to Dr. Gerald M. Jurica for his advice during my years at Texas Tech. Special thanks also go out to Debbie Kerr for her assistance in preparing this thesis, Mark Oberfield for his assistance in generating the computer programs, and to Teresa Bals for her moral support.

Finally, a special thanks goes to my wife, Becke for her understanding and support during the long hours I spent closed-in, writing text and drawing figures. Sincere thanks also go out to my parents, John and JoAnn Tumbiolo for their financial and moral support throughout my college years. Without this support, I could never have reached this educational goal. TABLE OF CONTENTS

ACKNOWLEDGMENTS ii LIST OF FIGURES v I. INTRODUCTION 1 1.1 Background 1 1.2 Data Sources 8 II. THE SYNOPTIC SETTING 12 2.1 17 October 1983 12 2.2 18 October 1983 23 2.3 19 October 1983 33 2.4 20 October 1983 43 2.5 21 October 1983 53 111. RESULTS AND ANALYSIS 64

3.1 Precipitation Analysis 18-21 October 64 3.2 Relative Humidity Analysis 17-21 October 96 3.3 Instability Analysis 17-21 October 102 3.4 Objective Analysis of Mixing Ratio 19-20 October. . . 104 3.5 Objective Analysis of Divergence 19-20 October ... 113 3.6 Objective Analysis of Moisture Convergence 19-20 October 132 3.7 Objective Analysis of Absolute Vorticity 19-20 October 137 3.8 Vertical Motion Analysis 19-20 October 145 3.9 Streamline Analysis 19-20 October 152

li IV. CONCLUSIONS 164

LIST OF REFERENCES 167

APPENDICES 168

A. RAWINSONDESTATIONSFORU.S. AND 169

B. DEFINITION OF K-INDEX AND LIFTED INDEX 170 LIST OF FIGURES

1.1. Number of Northeast Pacific tropical and hurricanes occurring between 1955-1986. . o 1.2. Accumulated precipitation totals for Texas and Oklahoma heavy rain event between 17-21 October 1983 4 1.3. Track of Hurricane Tico from 12-21 October 1983. Dark circles represent 0000 GMT times, open circles representl200 GMT times 6 1.4. (a) Monthly mean surface temperatures for the Eastern for October 1983 and (b) monthly deviation. Isotherms are in 'C 7 1.5. Tracks of Northeast Pacific hurricanes that produced heavv rains in Texas. From left to right; W (Waldo, 1985); P (Paine, 1986); N (Norma, 1981); D (Doreen, 1977); T (Tico, 1983); and N (Newton, 1986) 9 1.6. Rawinsonde network of 43 stations for U.S. and Mexico . . 11 2.1. a) Surface, b) 850 mb, and c) 700 mb synoptic analyses at 0000 GMT 17 October 1983. Striped areas represent low-level moisture, (surface dew point i 60'F, 850 mb dew point i 15'C) 14 2.2. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 17 October 1983 15 2.3. a) Surface, b) 850 mb, and c) 700 mb synoptic analysis at 1200 GMT 17 October 198J. Striped areas represent low-level moisture (Surface dew point i 60'F(15'C) and 850 mb dew points 15'C) 19 2.4. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 1200 GMT 17 October 1983 20 2.5. a) Surface, b) 850 mb, and c) 700 mb synoptic analyses at 0000 GMT 18 October 1983. Striped areas represent low-level moisture. (Surface dew point i 60'F(16'C) and 850 mb dew point H5'C) 24 2.6. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 18 October 1983 25

V 2.7. a) Surface, b) 850 mb, and c) 700 mb synoptic analyses at 1200 GMT 18 October 1983. Striped areas represent low-level moisture (surface dew point i 60*F (16*C) and 850 mb dew points 15'C) 29 2.8. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 1200 IBCJCtober 1983. Striped area represents 500 mb moisture (dew point i-10*C) 30 2.9. a) Surface, b) 850 mb, and c) 700 mb synoptic analyses at 0000 GMT 19 October 1983. Striped areas represent low-level moisture (surface dew point i 60*F(16'C) and 850 dew point i 15'C) 34 2.10. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 19 October 1983. Striped area represents 500 mb moisture (dew point i-10*C) 35 2.11. a) Surface, b) 850mb, andc) 700 mb synoptic analysis at 1200 GMT 19 October 1983. Striped represent low-level moisture (surface dew point i 60'F (16'C) and 850 mb dew point i 15'C) 39 2.12. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 1200 19 October 1983. Striped area represents 500 mb moisture (dew point i-lO'C) 40 2.13. a) Surface, b) 850 mb, and c) 700 mb synoptic analysis at 0000 GMT 20 October 1983. Striped areas represent low-level moisture (surface dew point i 60'F (16'C) and 850 mb dew point i 15'C) .... 44 2.14. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 20 October 1983. Striped area represents 500 mb moisture (dew point i -10'C). . 45 2.15. a) Surface, b) 850 mb, and c) 700 mb synoptic analysis at 1200 GMT 20 October 1983. Striped areas represent low-level moisture (surface dew point l 60"F (16'C) and 850 mb dew point i 10'C) 49 2.16. a) 500 synoptic analysis, and b) 200 mb isotach analysis at 12(J0 GMT 20 October 1983. Striped area represents 500 mb moisture (dew point i-lO'C) 50 2.17. a) Surface, b) 850 mb, and c) 700 mb synoptic analysis at 0000 GMT 21 October 1983. Striped areas represent low-level moisture (surface dew point i 60°F (16'C) and 850 mb dew point HO'C) 55 vi 2.18. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 21 October 1983. Striped area represents 500 mb moisture (dew point i-lO'C) . . 56 2.19. a) Surface, b) 850 mb, andc) 700 mb synoptic analysis at 1200 GMT 21 October 1983. Striped areas represent low-level moisture (surface dew point i60'F( 16'C) and 850 mb dew point nO'C) 60 2.20. a) 500 synoptic analysis, and b) 200 mb isotach analysis at 1200 GMT 2100 GMT 21 October 1983 61 3.1. The Texas-Oklahoma rain gauge network 65 3.2. Radar summary at a) 0135 GMT, b) 0435, GMT 17 October 1983 66 3.3. a) Radarsummary at 0135GMT 18 October, b) 0000 GMT to 1200 GMT 18 October observed rainfall (inches) 68 3.4. a) Radar summary at 1435 GMT 18 October, b) visible imagery at 1515 GMT 18 October 69 3.5. Radar summary at a) 2035 GMT and b) 2335 GMT 18 October 70 3.6. Observed rainfall (inches) from 0000 GMT 18 October to 0000 GMT 19 October 72 3.7. a) Radar summary at 0235 GMT 19 October b) enhanced infrared imagery at 0300 GMT 19 October 73 3.8. a) Enhanced infrared imagery at a) 0600 GMT and b) 0900 GMT 19 October 75 3.9. Radar summary at 1135 GMT 19 October 76 3.10. Observed rainfall (inches) from a) 0000 GMT to 1200 GMT 19 October and b) 0000 GMT 18 October to 1200 GMT 19 October 73 3.11. a) Visible imagery at 1431 GMT 19 October and b) Radar Summary 1435 GMT 19 October 79 3 12. a) Radar summary at 1735 GMT 19 October and b) visible imagery at 2031 GMT 19 October 81

vii 3.13. a) Enhanced infrared imagery at 2101 GMT 19 October andb) Radar summary at 2135 GMT 19 October 82 3.14. Enhanced infrared imagery at 2303 GMT 19 October 84 3.15. Observed rainfall (inches) from a) 1200 GMT 19 October to 0000 GMT 20 October and b) 0000 GMT 18 October to 0000 GMT 20 October 85 3.16. a) Enhanced infrared imagery at 0001 GMT 20 October and b) radar summary at 0135 GMT 20 October 87 3.17. a) Ehanced infrared imagery at 0300 GMT 20 October and b) radar summary 0435 GMT 20 October. . . 88 3.18. Enhanced infrared imagery at a) 0600 GMT and b) 0900 GMT 20 October 90 3.19. Radar summary at 1135 GMT 20 October 91 3.20. Observed rainfall (inches) from a) 0000 GMT to 1200 GMT 20 October and b) 0000 GMT 18 October to 1200 GMT 20 October 92 3.21. a) Enhanced infrared imagery at 1200 GMT 20 October and b) Radar summary at 1435 GMT 20 October 94 3.22. a) Visible imagery at 1835 GMT 20 October and b) Radar summary at 2035 GMT October 95 3.23. Observed rainfall (inches) from a) 1200 GMT 20 October to 0000 GMT 21 October and b) 0000 GMT 18 October to 0000 GMT 21 October 97 3.24. Cumulative rainfall (inches) from 0000 GMT 18 October to 0000 GMT 21 October for a) Lubbock (LBB) and b) Oklahoma City (OKC) 98 3.25. Radar summary at a) 0135 GMT and b) 1135 GMT 21 October 99 3.26. Surface-500 mb average relative humidity (%)/ precipitable water(inches) versus time for a) DRT, b) SEP, c) MAF, d) AMA, ande) OKC 100 3.27. K-lndex/Lifted Index versus time for a) DRT, b) SEP, c) MAF, d) AMA, and e) OKC 103 3.28. Grid system and upper air stations used in objective analysis 105 viji 3.29. Mixing ratio (g/kg) objective analyses for 0000 GMT 19 October at a) surface, b) 850 mb, c) 700 mb, and d) 500mb 106 3.30. Mixing ratio objective analyses for 1200 GMT 19 October at a) surface, b) 850 mb, c) 700 mb and d) 500 mb 108 3.31. Mixing ratio (g/kg) objective analyses for 0000 GMT 20 October at a) surface, b) 850 mb, c) 700 mb and d) SOOmb 110 3.32. Mixing ratio (g/kg) objective analyses for 1200 GMT 20 October at a) surface, b) 850 mb, c) 700 mb and d) 500 mb 112 3.33. Horizontal divergence (10"^ sec" ^) objective analyses for 0000 GMT 19 October at a) surface, b) 85C) mb, andc) 700 mb 114 3.34. Horizontal divergence (10'^ sec' ^) objective analyses for 0000 GMT 19 October at a) 50(J mb, b) 400 mb, c) 300 mb and d) 200 mb 115 3.35. Horizontal divergence (10"^ sec"') objective analyses for 1200 GMT 19 October at a) surface, b) Q56 mb and c) 700 mb 117 3.36. Horizontal divergence (10"^ sec" ^) objective analyses for 1200 GMT 19 October at a) 500 mb, b) 400 mb, c) 300 mb and d) 200 mb 119 3.37. Horizontal divergence (10"^ sec' ^) objective analyses for 0000 20 (Jctober at a) surface, b) 850 mb and c) 700mb 121 3.38. Horizontal divergence (10'^sec-1) objective analyses for 0000 GMT 20 October at a) 500 mb, b) 400 mb, c) 300mbandd) 2Q0mb 122 3.39. Horizontal divergence (10'^ sec'') objective analyses for 1200 GMT 20 October at a) surface, b) 85C) mb, and c) 700 mb 124 3.40. Horizontal divergence (10"^ sec'') objective analyses for 1200 GMT 20 October at a) 50Q mb, b) 400 mb, c)300mbandd) 200mb 125

IX 3.41. Vertical profiles of horizontal divergence (10'^ sec' ^) for MAF at a) 0000 GMT, b) 120(T GMT 19 October, c) 0000 GMT and d) 1200 GMT 20 October 127 3.42. Vertical profiles of horizontal divergence (10'^ sec-1) for AMA at a) 0000 GMT, b) 1200 GMT 19 October, c) 0000 GMT, and d) 1200 GMT 20 October 129 3.43. Vertical profiles of horizontal divergence (10"^ sec'^) for OKC at a) 0000 GMT, b) 1200 GMT 19 October, c) 0000 GMT, and d) 1200 GMT 20 Octobe 131 3.44. Surface moisture convergence (10'^ g/kg sec) objective analysis for 0000 GMT 19 October 133 3.45. Surface moisture convergence (10'^ g/kg sec) objective analysis for 1200 GMT 19 October 134 3.46. Surface moisture convergence (10'^ g/kg sec) objective analysis for 0000 GMT 20 October 136 3.47. Surface moisture convergence (10"^ g/kg sec) objective analysis for 1200 GMi 20 October 138 3.48. 500 mb absolute vorticity (10'^ sec' ^) objective analysis for 0000 GMT 19 October 139 3.49. 500 mb absolute vorticity (10'^ sec'M objective analysis for 1200 GMT 19 October 141 3.50. 500 mb absolute vorticity (10'^ sec" ^) objective analysis for 0000 GMT 20 October 142 3.51. 500 mb absolute vorticity (10'^ sec" ^) objective analysis for 1200 GMT 20 October 144 3.52. 500 mb vertical motion (|xb/sec) analysis for 0000 GMT 19 October 146 3.53. 500 mb vertical motion (|xb/sec) analysis for 1200 GMT 19 October 147 3.54. 500 mb vertical motion (|xb/sec) analysis for 0000 GMT 20 October 149 3 55. 500 mb vertical motion (|ib/sec) analysis for 1200 GMT 20 October 151 3.56. Streamline objective analysis for 0000 GMT 19 October at a) surface, b) 850 mb, andc) 700 mb 153

X 7 3.57. Streamline objective analysis for 0000 GMT 19 October at a) 500 mb, b) 400 mb, c) 300 mb and d) 200 mb. . . . 154 3.58. Streamline objective analysis for 1200 GMT 19 October at a) surface, b) 850 mb, andc) 700 mb 155 3.59. Streamline objective analysis for 1200 GMT 19 October at a) 500 mb, b) 4000 mb, c) 300 mb and d) 200 mb . . 156 3.60. Streamline objective analysis for 0000 GMT 20 October at a) surface, b) 850 mb, andc) 700 mb 158 3.61. Streamline objective analysis for 0000 GMT 20 October at a) SOOmb, b) 400mb, c) 300 mb and d) 200 mb ... 159 3.62. Streamline objective analysis for 1200 GMT 20 October at a) surface, b) 850 mb, andc) 700 mb 161 3.63. Streamline objective analysis for 1200 GMT 20 October at a) SOOmb, b) 400mb, c) 300 mb and 200 mb .... 162

XI CHAPTER

INTRODUCTION

The Northeast Tropical Pacific was long unrecognized as a major source region for tropical . Since the availability of satellite imagery, this region has emerged as the second-ranking area in the world, behind the Northwest Tropical Pacific, for tropical occurrence. Figure 1.1 shows the number of tropical storms and hurricanes that occurred per year in the Northeast Tropical Pacific from 1955-1986. An increase in the number of cyclones per year was recognized after 1965, due to the availability of satellite imagery. From the period 1966-1980, a total of 214 cyclones of at least tropical strength (winds greater than 34 kt) were documented (Allard, 1984). Of that number, 30 (14%) recurved and made landfall upon the west coast of Mexico. Heavy rain events which have occurred over the southwestern U.S., including Texas and Oklahoma, have been attributed to these landfalling tropical cyclones. This thesis is the analysis of one such heavy rain event that took place over the South Plains of Texas and a large portion of Oklahoma and . This particular event was attributed to Northeast Pacific Hurricane Tico as it passed through this region.

11 Background Since 1980 there have been six observed heavy rain events over Texas and Oklahoma due to the influx of remnants of tropical storms and ^5

20

IS

10

55 60 65 10 15 iO SS

Figure 1,1. Number of Northeast Pacific tropical storms and hurricanes occurring between 1955-1986. hurricanes originating in the Northeast Pacific. Hurricane Norma (1981), Hurricane Tico (1983), Hurricane Waldo (1985), and Hurricanes Newton, Paine, and Roslyn (1986) each contributed to significant rain events in this region. All of these events took place during the months of September and October. During the autumn months September, October, November of the 1980's , 500 mb long-wave troughs located off the west coast of the U.S. were common (National Meteorological Center (NMC) 500 mb charts 1980-1986). This pattern often resulted in recurving tropical cyclones, which make landfall on the west coast of Mexico and continue to travel into Texas and Oklahoma. There have been several case studies of heavy rain events that have occurred well inland for Atlantic tropical cyclones that have made landfall on the East and Gulf coasts of the U.S. A well-documented case was Hurricane Agnes with heavy rains in southwestern New York and northwestern Pennsylvania several days after landfall on the panhandle of Florida (Bosart and Carr, 1977). This may well be the first in-depth study of a heavy rain event occurring well inland of a Pacific landfalling . During the period 17-21 October 1983, the effects of Hurricane Tico were felt in the South Central states. In a 150 km-wide band which extended from the South Plains of Texas near Lubbock through Oklahoma and into central Missouri, heavy precipitation well in excess of 10 inches was observed as the remnants of Northeast Pacific Hurricane Tico passed through the area. Figure 1.2 shows the accumulated precipitation totals for this heavy rain event (Storm Data, October. 1983). The circulation of Tico Figure 1.2. Accumulatedprecipitation totals for Texas and Oklahoma heavy rain even between 17-21 October 1983. Isohyets are in intervals of one inch. (After Storm Data, Oct. 1983) was believed to have played a major role in this event despite having traveled over 2000 km from its landfall upon the west coast of Mexico near the city of Mazatlan. Tico, first known as tropical depression number twenty, developed in the Northeast Pacific on 12 October 1983 near 8 N 102 W longitude. Figure 1.3 shows the track of Hurricane Tico. While moving in a northward direction, the depression intensified and became Tropical Storm Tico on 13 October. Tico continued to intensify and reached hurricane strength on 15 October with sustained winds of 80 kt near 16 N 104 W. At this time, the hurricane turned and began heading in a west-northwesterly direction. This was mainly due to upper-level high pressure over central Mexico and warmer sea surface temperatures off the west coast of Mexico. Figures 1.4 shows the monthly mean (sea surface temperatures) and monthly deviation from the mean, respectively, for the Eastern Pacific Ocean in October 1983. Noted were the positive temperature anomalies (and more growth) off the western coasts of the U.S. and Mexico. Hurricane Tico reached its maximum intensity on 18 October as its winds exceeded 115 kt. The hurricane's position at this time was 18 N 110 W. Due to an upper-level trough situated over the southwestern U.S., Hurricane Tico suddenly recurved to the northeast and headed toward the west coast of Mexico. Tico continued to head in a northeastward direction and made landfall on the west coast of Mexico, near the city of Mazatlan, on 19 October at approximately 1400 GMT. After landfall, Tico continued to travel in a northeast direction, crossing the Sierra Madre Occidental mountain range, while maintaining its cyclonic circulation. During this lONGnUO€

Figure 1.3. Track of Hurricane Tico from 12-21 October 1983. Dark circles represent 0000 GMT times, open circles represent 1200 GMT times. ••»•»

tmt i««B M*B *— **» '*»

Figure 1.4. (a) Monthly deviation from mean surface temperatures for the Eastern Pacific Ocean for October 1983 and(b) monthly mean. Isotherms are in'C. 8 time, the translational speed of Tico increased, mainly due to a 120 kt jet at 200 mb. At 1200 GMT on 20 October, the remnants of Hurricane Tico entered southwest Texas, near the city of Del Rio. Shortly after this time, the storm began to take on extratropical characteristics. The low pressure center continued to head towards the northeast through Texas and was near Oklahoma City on 21 October at 0000 GMT, leaving behind a swath of heavy precipitation. The track of Hurricane Tico differed from the tracks of other recent Northeast Pactific tropical cyclones. Figure 1.5 shows the tracks of Northeast Pacific hurricanes that produced heavy rains in Texas. Note how the track of Tico (T) differed from those of Waldo (W, 1985), Paine (P, 1986), Norma (N, 1981), Doreen (D,1977), and Newton (N, 1986). The area between the cities of Los Mochis and Mazatlan seemed to be the prime landfalling region for tropical cyclones that eventually produced heavy rains in west Texas.

1.2 Data Sources The Northeast Tropical Pacific, as defined in this study, is the area bounded by the west coast of North America, extending westward to 140 W, between 5 N and 40 N. A previous study (Allard, 1984) looked at the climatological aspects of cyclones occurring in this region. Prior to 1965 observations in this region were mainly from ships and buoys. Since that time, satellite imagery has taken over the primary task of monitoring this region. lONGOUOC Figure 1.5. Tracks of Northeast Pacific hurricanes that produce heavy rains in Texas. From left to right, W (Waldo, 1985); P (Paine, 1986); N (Norma, 1981); D (Doreen, 1977); T (Tico, 1983); and N (Newton, 1986).

Ak 10 The primary sources of data for this research consisted of standard National Meteorological Center (NMC) surface and upper-air maps and rawinsonde observations for 43 stations in the U.S. and Mexico. The time period of this is 17-21 October 1983. Upper-air charts (850 mb, 700 mb, 500 mb, 300 mb, and 200 mb) were obtained for this time period twice daily (0000 GMT, and 1200 GMT). Surface maps were obtained at 3-hour intervals during this time period. Rawinsonde observations on microfiche were obtained from the National Climatic Data Center in Asheville, North Carolina. The data consisted of twice daily soundings (0000 and 1200 GMT) for 38 stations in the U.S. and 5 stations in Mexico, for a total of 43 stations. Additional Mexican data came from surface reports every 3 hours, although not every station reported each observation. Figure 1.6 shows the rawinsonde network used in this research. Precipitation data for 81 recording stations in Texas and Oklahoma were acquired from monthly publications of rainfall from the National Climatic Data Center (Hourly Precipitation Data, October 1983). These data were used to perform 12-hour precipitation analyses in Texas and Oklahoma from 17-21 October. / / \ \

(^a£ .€^\p

MET

Figure 1.6. Rawinsonde network of 43 stations for U S and Mexico. CHAPTER 11

THE SYNOPTIC SETTING

This study focuses on tropical cyclone Tico, which produced a band of heavy precipitation, well downstream of its landfall position. Along its post-landfall track, the tropical cyclone entered a highly baroclinic region in Texas and Oklahoma. This interaction resulted in the transformation of the cyclone from tropical (warm core) to extratropical (cold core) in nature. From 17-21 October, a series of synoptic and mesoscale features, including the remnants of Hurricane Tico, came together over west Texas and south central Oklahoma, resulting in the heavy rain event over this region. This chapter takes an in-depth look at the surface and upper-level synoptic conditions which led to the heavy precipitation.

2.1 17 October 1983 On 17 October Hurricane Tico was located off the west coast of Mexico, east-southeast of the city of Manzanillo near 17 N 108 W. At this time Tico was traveling towards the west-northwest with sustained winds of 105 kt. Synoptic conditions over eastern New Mexico, western Texas, and Oklahoma were already producing thunderstorm activity in this region.

12 13

2.1.1 17 October 0000 GMT Figures 2.1a., b., c, and 2.2a., b. show the synoptic patterns at the surface (sfc), 850 mb, 700 mb, 500 mb, and .the isotach pattern at 200 mb, respectively. Surface conditions at this time consisted of a cold front extending from a 1000 mb low pressure center over central Canada, southward through the Great Lakes states, and then curving towards the southwest through Missouri, Oklahoma, and the South Plains of Texas, while becoming stationary over this region. Behind the front, high pressure, with a 1024 mb center in South Dakota, dominated the Central Plains states. An inverted low pressure trough, oriented southwest-northeast, was located along the stationary front from southwest Texas through Oklahoma and into southwest Missouri. South of the stationary front, temperatures ranged from 24'C to 28'C with dew point temperatures ranging from 13'C along the front to 22'C near the Texas Gulf Coast. Winds in this region were generally from the southeast at around 10-15 kt. North of the front, temperatures ranged from the upper 50'F to the low 60'F, with dew points mainly in the 40'F. Winds in this region were from the northeast at 10 kt, indicating moderate surface convergence along the front. At this time, thunderstorm activity was occurring in eastern New Mexico and western Texas, north of the stationary front. Earlier in the day, squall line thunderstorms produced over 1.5 inches of rain in south central Oklahoma. At 0000 GMT, Lubbock (LBB) was reporting a thunderstorm in progress, with a temperature of 75'F (24'C), and a dew point of 54T 14

\^-1

Figure 2.1. a) Surface, b) 850 mb, and c) 700 mb synoptic anayses at 0000 GMT 17 October 1983. Striped areas represent low-level moisture, (surface dew point i 60'F, 850 mb dew point i 15'C). 15

,„'no?^.»?»-^^«-'''^- 16

(12'C), and southeast winds at 10 kt. The surface pressure at Lubbock was 1012 mb and had been falling during the past 3 hours. Conditions at the 850 mb level consisted of high pressure over the southeastern U.S. with a 1585 m center located in North Carolina. Two low pressure areas along with a negatively tilted ridge were located over the western half of the U.S. These features resulted in strong, southeasterly flow into west Texas, and along with it an abundant supply of low-level moisture from the Gulf of Mexico. At 850 mb, dew points of 15'C (59'F) extended to north of Austin, dew point temperatures in the South Plains region were around 10'C (50'F). The 850 mb temperature pattern consisted of a pocket of warm air (25'C) located in northern Mexico, south of the El Paso (ELP) region. Also, warm air was being advected into west Texas and the southern Rocky Mountains region, ahead of a developing low pressure trough. Cold air, in association with the trough, was being filtered south along the west coast of the U.S. At this time, height falls of 10-20 m were being recorded throughout the southwestern states, evidence of developing low pressure. At 700 mb, high pressure was dominated over the southeastern states and the Gulf of Mexico. A broad low pressure trough with embedded short waves dominated the western half of the U.S. At this time, one short wave was traveling through northern Mexico and into southwestern Texas. Other short waves were moving through Arizona and Colorado. The main trough was located along the west coast. South to southwesterly flow was prevalent throughout the southwestern states with speeds ranging from 10 17 to 25 kt. Amarillo (AMA), Midland-Odessa (MAF), and Oklahoma City (OKC) were reporting winds from the southwest at 20 kt, with slight diffluence occurring over the Texas Panhandle. The temperature pattern at 700 mb indicated warm air advectlon occurring over west-central Texas and Oklahoma from northern Mexico. Temperatures in the region ranged from 5'C at OKC to 13'C at Chihuahua (CUU). At this time, there were no signs of significant 700 mb moisture advectlon in the area. Height rises of 10-20 m were common throughout the southwest. Conditions at 500 mb were similar to those at 700 mb. The Bermuda high (5910 m center) dominated the Gulf of Mexico. A broad low pressure trough was anchored over the west coast, with minor short wave disturbances downstream of the main trough. Moderate southwesterly flow of 30-40 kt was common throughout Texas and Oklahoma. Speed divergence was evident over eastern New Mexico. This southwesterly flow was an indication that the wind field was veering with height because low-level (sfc, 850 mb) winds were from the southeast. Southerly wind flow in the lower-levels, combined with veering winds in the mid-and upper-levels, have been observed with many significant rain events (Grice and Maddox, 1982; Johnson and Mortimer, 1983). The 500 mb temperature pattern revealed a short wave in the -15'C isotherm advecting through New Mexico at this time; 500 mb temperatures in the region ranged from -12'C to -15'C. The upper-level divergence and cold air advectlon helped support the thunderstorm activity that was presently occurring over eastern New Mexico . Mid-level dry air (>30'C dew ]8 point depression) was being advected into southwestern Texas at this time. Height rises of 10-50 m were common throughout the southwest. The most significant height falls were confined to the extreme northwestern portion of the U.S. The polar jet stream at 0000 GMT was located along the west coast, and from central Mexico to the Great Lakes. The 200 mb isotach pattern showed 70 kt winds stretching from the Sierra Madre Occidental mountain range in central Mexico to central Michigan, with a portion of the jet axis branching off to the southeast; 90 kt winds were recorded over Del Rio (DRT) and Longview, Texas (GGG). Wind speeds in the Amarillo-Midland region were 50-60 kt.

2.1.2 17 October 1200 GMT Figures 2.3a. b., c, and 2.4a., b. show the synoptic patterns at the 5 pressure levels, respectively, 1200 GMT 17 October. Little change took place at the surface over the past 12-hours. A stationary frontal boundary stretched from the Ohio Valley states to central Texas, slightly south of its previous 12-hour position. High pressure (1026 mb) over Iowa has traveled eastward. In Texas, moist, southeasterly winds continued south of the frontal zone. Temperatures were near 22'C, with dew points 18'C to 21 'C. North of the front, northeast winds prevailed, with temperatures and dew points from lO'C to 16'C. Many portions of eastern New Mexico and western Texas were experiencing dense fog at this time. Rain showers were occurring along the stationary front in north-central Texas and 19

FiQure 2 3. a) Surface, b) 850 mb and c) 700 mb dew po1ntT50'F(15-C) and 850 mb dew point l\5C) 20

Finure 2 4 a) 500 mb synoptic analysis, and b) 200 Xsotach analysis at'l 200 GMT l) October 1983. 21

Oklahoma. At 1200 GMT, Lubbock reported fog with a temperature and dew point of 54'F (12'C), a north wind at 5 kt, and a surface pressure of 1017 mb and rising. High pressure continued to dominate the southeast at 850 mb. A broad, negatively tilted ridge extended westward to the Rocky Mountains. Southeast winds (10-20 kt) continued to pump low-level moisture into west Texas. A tongue of moist air (15'C isodrosotherm) extended up through the Concho Valley and Permian Basin. The South Plains region was also experiencing strong warm air advectlon. Temperatures at 850 mb ranged from 9'C at Amarillo (AMA) to 16'C at DRT. Twelve-hour height rises of 10-20 m were common in the area, with height falls of 10-30 m occurring in the extreme southwestern U.S. and northern Mexico, in the location of two low pressure regions. At 700 mb, a trough was located over the southwest, with an associated low in southern . High pressure continued to influence the southeast. Southwesterly flow (10-15 kt) was common throughout New Mexico, Texas and Oklahoma. At this time, a couple of short waves, downstream of the main trough, were traveling through Arizona and Colorado. Warm-air advection was taking place over west Texas, with temperatures ranging from 4'C at AMA, to 8'C at MAF, to 11'C at CUU. Warm-air advection at this level was indicative of a stabilizing atmosphere, surely not what was expected in a heavy rain event case. Also, no significant moisture advection was occurring at 700 mb in the three-state region (New Mexico, Texas, and Oklahoma) at 1200 GMT. 22

At 500 mb, a deepening low pressure trough was located over the west coast, with the main trough line extending from Washington to Baja, California. Twelve-hour height falls of 20-60 m in southern California were an indication that the trough continued to develop to the south. A short wave, located over the Permian Basin, was advecting through the region at this time. Twelve-hour height falls of 10-30 m throughout Texas and Oklahoma were a reflection of the presence of the short wave. Winds in the three-state region varied from the southwest to the west-northwest at 30-40 kt. Upper-level diffluence was located over north-central Texas and south-central Oklahoma. The general westerly wind flow pattern at this level was still an indication of winds veering with height. The 500 mb temperature pattern showed cold air continuing to filter southward along the west coast, in association with the deepening trough. Slight warm air advection was occurring over eastern New Mexico and western Texas, with temperatures ranging from -12'C at OKC to -8'C at ELP. Very dry air was present over the region with no mid-level moisture advection taking place. No major changes took place in the position of the polar jet stream. The 200 mb isotach pattern showed two main areas of the jet. One portion of the jet, in association with the trough, was located over the southwest, with the 70 kt isotach stretching into northeast Arizona. The other main portion extended from southwest Texas near DRT eastward along the Gulf Coast states. The 200 mb winds in the AMA-LBB region were from the west at 60 kt. 23

2.2 18 October 1983 On 18 October Hurricane Tico was located at 17 N 111 W, its westernmost position. At this time, the storm was also at its most intense stage, with sustained winds exceeding 115 kt. Soon after, Tico began to recurve under the influence of the low pressure trough over the eastern Pacific Ocean and western U.S.

2.2.1 18 October 0000 GMT Figures 2.5a., b., c, and 2.6 a., b show the synoptic patterns at the surface, 850 mb, 700 mb, 500 mb, and 200 mb, respectively. The surface cold front and high pressure system continued traveling towards the east at 0000 GMT. The frontal boundary extended from upstate New York, through the Mississippi Valley, and into central Texas. High pressure (1025 mb) was located over the Great Lakes. The stationary front over central Texas had become a warm front, and had slowly drifted to the north. Southeasterly flow persisted in central and south Texas, with temperatures in the 70'F and 80'F and dew points in the upper 60'F to low 70'F. North of the front, the winds over west Texas had veered around to the east-southeast, an indication of overrunning conditions along the frontal zone. The winds in Oklahoma had continued to be from the northeast. Temperatures north of the front ranged from 17'Cto24'C. An increase in the surface moisture had occurred along the front over the 24

Figure 2.5. a) Surface, b) 850 mb, and c) 700 mb synoptic analyses at 0000 GMT 18 October 1983. Striped areas represent low-level moisture. (Surface dew point l 60'F (16'C) and 850 mb dew point l 15'C) 25

Figure 2 6 a) 500 mb synoptic analysis, and b) 200 mo isotach analysis at (JOOO GMT 18 October 1983. 26 front, with dew points rising to 17'C. Thunderstorm activity had occurred along the frontal boundary. This region had received nearly an inch of rainfall over the past 12-hours. The overall pattern at 850 mb had not changed significantly. High pressure continued to influence the east coast. A generally unorganized pattern, with low pressure areas was over the western half of the U.S. At this time, a north-south oriented short wave was advecting through west Texas. Twelve-hour height falls from Texas to Arizona were around 20-30 m, consistent with the presence of a short wave. South-south­ easterly flow remained throughout all of Texas. Low-level moisture continued to enter west Texas. Strong warm-air advection also became persistent over the region, with temperatures ranging from 12'C at AMA to 20'C at MAF. The moisture and.warm-air advection resulted in the destabilization of the environment in the vicinity of the surface front (Byrd and McCaul, 1985). At 700 mb, a trough was located over the southwestern states, with an associated closed low just south of San Diego (SAN). The trough had deepened and continued its southward development. A series of short waves, originating over western Mexico, were traveling through the southwestern states at this time, but no significant height changes were observed over the region. Southwesterly flow of 15-25 kt had continued Texas, Oklahoma, and New Mexico, with slight 700 mb diffluence occurring over eastern New Mexico. Moderate warm air advection was still taking place in eastern New Mexico and western Texas. Temperatures ranged from 27

6'C at AMA to 12'C at CUU. At 0000 GMT, there was mid-level moisture present near MAF although there was no evidence of moisture advection. The pattern at 500 mb was very similar to the pattern at 700 mb. A deepening trough was located over the southwest, with an associated closed low near SAN. Twelve-hour height falls of 30-60 m were observed from SAN to Guadalupe Island, Mexico (GUP), to Tucson, Arizona (TUS). A series of short waves were located throughout the southwest, with one over west Texas at this time, although 12-hour height rises of 10-40 m were common throughout the region. West-southwesterly flow (30-40 kt) persisted over the three-state area, with 500 mb diffluence occurring over southwest Texas, and slight speed divergence taking place over eastern New Mexico. Winds continued to veer with height over the three-state area. The temperature pattern showed continuing warm-air advection over southwest Texas and slight cold-air advection in the area of the speed divergence. Temperatures ranged from -11'C at Albuquerque, NM (ABQ) to -6'C at CUU and -5'C at Monterrey, Mexico (MTY). At 0000 GMT, no mid-level moisture was evident at 500 mb. The 200 mb polar jet stream changed over the previous 12-hours. The jet intensified over the southwestern states in association with the west coast trough. The 200 mb isotach pattern showed a band of 70 kt winds stretching from the Eastern Pacific to southern South Dakota, with the entire Texas Panhandle and South Plains included in this band. A band of 90 kt winds had developed, extending from Baja California to the Central Plains, with the Texas and Oklahoma Panhandle region located in the 28 right-front quadrant of the 90 kt jet. Diffluence at 200 mb was occurring over eastern New Mexico at this time.

2.2.2 18 October 1200 GMT Figures 2.7a., b., c, and 2.8a., b show the synoptic patterns at the five pressure levels at 1200 GMT on 18 October. Surface conditions over Texas, Oklahoma, and New Mexico had not changed dramatically over the previous 12-hours. Overall, the cold front and high pressure center over the northern U.S. continued its eastward movement, with a lingering warm/stationary front positioned over central Texas. Moist, southeasterly winds continued south of the frontal zone in south Texas. Temperatures and dew points in this region were in 20'C. Over west Texas, southeast winds persisted from MAF to AMA, with temperatures and dew points ranging from 13'C to 18'C. At 1200 GMT, Lubbock reported a temperature and dew point of 61'F (16'C), with light rain. Winds were southeast at 10 kt, with a falling pressure 1015 mb. Over Oklahoma, northeast winds continued with widespread rainshower and thunderstorm activity. A rather unorganized pattern at 850 mb remained, with high pressure influencing the eastern half of the U.S. and low pressure over the western half. This combination resulted in the continuation of moderate southeasterly flow (20-25 kt) into Texas and Oklahoma. Low-level moisture advection increased throughout the area, with the 15'C isodrosotherm extending through the South Plains and into central Oklahoma. At this time, an 850 mb short wave was still situated over west 29

Fiqure 2.7. a) Surface, b) 850 mb, and c) 700 mb svnoDtic analyses at 1200 GMT 18 October 1983 Strioed areas represent low-level moisture (surface dew point i 60'F (16'C) and 850 mb dew points 15'C). 30

Figure 2.8. a) 500 mb synoptic analysis, and b) 200 mt) isotach analysis at 1200 18 October 1983. Striped area represents 500 mb moisture (dew point i -10'C). 31

Texas. Twelve-hour height falls of 10-20 m were observed over the Texas Panhandle and Oklahoma. Strong warm air advection continued in the region with temperatures ranging from 13'C at AMA to 17'C at DRT. The moisture and warm air advection at 850 mb continued the destabilization of the atmosphere at this level. The trough and low pressure center at 700 mb, located over the west coast. Intensified slightly and developed towards the south during the previous 12-hour period. The low pressure center, which had been previously positioned over SAN, moved near Guadalupe Island. Twelve-hour height falls of 10-30 m were recorded throughout the southwestern states, with maximum height falls over northern Mexico, from Empalme (EMP) to CUU. This indicated that the trough continued to develop to the south-southeast. A minor short wave was traveling through the southern Rockies at this time. South-southwesterly flow (15-30 kt) continued in the three-state region. The 700 mb temperature pattern still showed warm air advection over eastern New Mexico and western Texas, with temperatures ranging from 4'C at AMA to 11'C at ELP and 10'C at CUU. There were signs of increasing mid-level moisture from CUU to OKC, with dew point depressions decreasing slightly throughout the area. The pattern at 500 mb was very similar to the pattern at 700 mb. The trough and low pressure center had intensified slightly and moved slowly to the south-southeast, with the center in northern Baja California. Maximum height falls 40 m were in northern Mexico, 12-hour with height falls of* 10-30 m common throughout the rest of the southwest. Moderate 32 southwesterly flow of 30-40 kt continued in the region, as winds continued to veer with height. Very slight speed divergence was also evident over west Texas and Oklahoma. Changes in the 500 mb temperature pattern had begun to take place, with more substantial changes occurring in the moisture field. Cold-air advectlon was now observed over eastern New Mexico and the Texas Panhandle, with warm-air advection continuing to occur in southwest Texas. Cold-air advection at this level, along with warm-air advection at the lower levels, destabilizes the atmospheric layer as a whole. The isodrosotherm pattern showed abundant mid-level moisture over central and northern Mexico (CUU), with increasing moisture levels in Texas and Oklahoma as well. Mid-level moisture advection, from the Northeast Pacific Ocean, took place throughout west Texas. Mid-level moisture content would continue to increase over this region during the next 24-48 hours. The southwest-northeast oriented polar jet stream at 200 mb continued to expand in areal coverage. The band of 70 kt winds now extended from off the coast of Baja California to the upper Great Lakes region. The band of 90 kt winds now stretched from the west coast of Mexico to the Central Plains states, with 100 kt winds recorded at Dodge City, KS (DDC). Slight 200 mb diffluence was occurring over southeast New Mexico and southwest Texas at 1200 GMT. 33

2.3 19 October 1983

On 19 October, Hurricane Tico was nearing the west coast of Mexico; eventually making landfall near the city of Mazatlan at 1400 GMT. Widespread wind and damage took place as the storm moved on shore. An estimated $66 million worth of damage and nine fatalities occurred in this area. On this day, heavy rains began from west Texas to central Oklahoma. During the next 24 hours, over five Inches of rain would fall from LBB to OKC as Tico moved into Mexico.

2.3.1 19 October 0000 GMT Figures 2.9 a., b., c, and 2.10 a., b show the synoptic patterns at the surface, 850 mb, 700 mb, 500 mb, and 200 mb, respectively at 19 October 0000 GMT. At the surface, a new area of low pressure (1006 mb) had formed over southeast Colorado. The quasi-stationary front positioned in central Texas had again become a warm front and moved northward into the Texas-Oklahoma Panhandle, under the influence of the surface low. Southeasterly winds throughout Texas and Oklahoma brought an abundant supply of Gulf moisture into the region. Dew point temperatures over a majority of the region were near 18'C, with 13'C confined to the AMA area. Temperatures ranged from 17'C in the Panhandle to 29'C in south Texas. At this time, widespread rainshower and thunderstorm activity was occurring from CUU to OKC. At 0000 GMT, LBB reported a thunderstorm in progress, with a temperature and dew point temperature of 72'F (22'C) 34

Figure 2.9. a) Surface, b) 850 mb, and c) 700 mb synoptic analyses at 0000 GMT 19 October 1983. Striped areas represent low-level moisture (surface dew point i 60'F (16'C) and 850 dew point i 15'C). 35

Fiaure 2 10 a) 500 mb synoptic analysis, and b) 200 m?) isotach analysis at 0600 GMT 19 6ctober 1983. Striped area represents 500 mb moisture (dew point i -10'C). 36 and 64'F (18'C), respectively. Winds were southeast at 10 kt, with a falling surface pressure of 1010 mb. The pattern at 850 mb had become somewhat more organized from the previous 12-hours. High pressure continued to dominate the eastern half of the U.S. Minor short waves on the western side of the 850 mb ridge passed through Texas and Oklahoma, and more organized area of low pressure was located from the southwestern states into the central Rockies. Twelve-hour height falls of 10-30 m were observed over Texas and Oklahoma, in association with the short waves. Moderate southeasterly flow persisted in the region, with AMA reporting northwest winds, an indication of 850 mb convergence over the northern sections of the South Plains. Low-level moisture continued to flow into Texas and Oklahoma, with the 15'C isodrosotherm extending into northern Oklahoma. The strong warm-air advection over the area had decreased somewhat with temperatures ranging from 16'C at OKC to 19'C at MAF; but a pocket of warm air (20'C) was located just west of a region of heavy rain in New Mexico and Arizona. At 700 mb, a broad trough and closed low dominated the southwest, with a secondary trough situated over the northern Rockies. The closed low over Baja California intensified slightly and slowly traveled in a southerly direction. Maximum 12-hour height falls of 40 m were still located in northern Mexico (EMP, CUU), with 12-hour height falls of 10-20 m over Texas and Oklahoma. Winds at this level were from the south-southeast in south Texas, veering to the southwest and west over the Texas Panhandle 37 and Oklahoma. Wind speeds in this region were 20-25 kt. Diffluence at 700 mb occurred throughout west Texas at this time. The temperature pattern showed moderate cold-air advection taking place over western Oklahoma, while warm-air advection was evident in eastern New Mexico. Temperatures ranged from 4*C at AMA, to 8'C at ABQ. Moisture advection at 700 mb continued over west Texas, with increased moisture levels at this time.

At 500 mb, the trough and closed low over the southwest followed a similar pattern. This system drifted slowly towards the south-southeast. A 12-hour maximum height fall of 60 m was now observed at EMP, suggesting continued eastward movement. Twelve-hour height falls of 10-40 m were common throughout the rest of the region. Southwesterly flow of 30-40 kt persisted over the three-state area, as well as northern Mexico, while winds of 65 kt were recorded at EMP. Slight diffluence occurred over southeastern New Mexico and the Texas-Oklahoma Panhandle region, as winds continued to veer with height. No significant changes took place in the 500 mb temperature pattern. Cold-air advection continued over eastern New Mexico. At this time, an abundant supply of Pacific moisture (Tico) advected into west Texas, as the -10'C isodrosotherm extended to the Texas Panhandle. The onset of the heavy precipitation over Texas and Oklahoma coincided with the arrival of this mid-level moisture. The polar jet stream continued to intensify. The 200 mb isotach pattern showed the band of 70 kt winds stretching from Baja California to the New 38

England states. The band of 90 kt winds extended from Mexico to the Great Lakes states, while a band of 110 kt winds developed over and the Texas-Oklahoma Panhandle, with a 120 kt maximum over DDC. The heavy rain area was located underneath the right-rear quadrant of the 110 kt jet, In a region of positive vorticity advection (PVA). It was also the intensification of the polar jet that contributed to the mid-level moisture advection into the heavy rain region.

2.3.2 19 October 1200 GMT Figures 2.11 a., b., c, and 2.12 a., b show the synoptic patterns at the five pressure levels at 19 October 1200 GMT. Hurricane Tico was close to landfall near Mazatlan. At the surface, the low pressure center and cold front, previously in southeast Colorado, moved into west Texas, with, the center situated over the AMA area and the cold front south of LBB and MAF. The warm front continued northward into extreme southern Kansas. An inverted trough oriented SW-NE was also located over west Texas. Slightly cyclonic circulation, along with surface convergence could be detected in the vicinity of the low and cold front. Warm moist air from the Gulf of Mexico continued to flow into the region, while cool, nearly saturated air began to enter extreme western Texas behind the cold front. At 1200 GMT, LBB had moderate rain, temperature and dew point both 60'F (16'C), east winds at 10 kt, and a rising surface pressure of 1010 mb, indicating frontal passage. During the previous 12-hours, heavy rain fell from the Permian Basin to south-central Oklahoma. Heaviest precipitation occurred over 39

Fiqure 2.11. a) Surface, b) 850 mb, andc) 700 mb synoptic analysis at 1200 GMT 19 October 1983. Striped represent low-level moisture (surface dew point 1 60'F (16'C) and 850 mb dew point i 15'C). 40

Figure 2.12. a) 500 mb synoptic analysis, and b) 200 mo isotach analysis at 1200 19 October 1983. Striped area represents 500 mb moisture (dew point i -10'C). 41 extreme southwestern Oklahoma and north-central Texas, with over 4 Inches recorded. LBB received over 1.5 Inches of rain during this time. Conditions at the 850 mb level changed little during the 12-hour period. A large area of low pressure dominated the entire southwest, with slight intensification. Twelve-hour height falls of 10-30 m continued to be observed throughout the area. South-southeasterly flow continued the advectlon of warm, moist air into the region, but the tongue of low-level moisture (15'C isodrosotherm) began to shift eastward. At 1200 GMT, the wind pattern over the Central Plains (northerly flow) suggested that cold-air advection was beginning in southern Kansas and northern Oklahoma. This trend of cold-air advection into the heavy rain region would continue over the next 24-48 hours. The closed low at 700 mb over Baja California moved slowly eastward and was located over northwest Mexico. No intensification of the the center took place in this time period. Maximum height falls in northern Mexico and southwest Texas suggested east-northeastward movement over the next 24 hours. South-southwesterly flow continued at this level, with slight diffluence over west Texas. No significant changes occurred in the temperature and moisture fields, with continued presence of moisture at 700 mb in the three-state region. The 500 mb low also showed slight eastward movement. Twelve-hour maximum height falls of 40 m at TUS and CUU indicated continued movement eastward. An inverted short wave in northeast New Mexico caused upper-level diffluence over western Texas. Strong southwesterly 42 winds (35-50 kt) continued throughout the area, with speed divergence occurring between MAF and OKC. Winds also continued to veer with height over this region. An interesting pattern has been revealed in the 500 mb temperature field. A tongue of warm, mid-level air (-5'C), extending from central Mexico to near DRT, appeared at the time when Hurricane Tico was making landfall upon the west coast of Mexico. This feature, to an even greater extent, has been noticed in more recent cases, namely Hurricanes Newton and Paine (1986). In these cases, very warm 500 mb temperatures (-5'C or warmer) advected well downstream into Texas as the storms made landfall. In the case of Hurricane Paine, an unusually warm 500 mb temperature of -3'C was recorded at MAF at the time of landfall. Mid-level moisture from the Pacific associated with Tico's circulation continued to flow into the region, with an expansion in the areal coverage of the moist tongue (-10' C isodrosotherm). This moist tongue extended into the northern portions of the Texas Panhandle. The polar jet stream changed little. The band of 110 kt winds shifted northeast somewhat, and was located over and Indiana, but 90-100 kt winds continued throughout the three-state region, with Texas and Oklahoma located on the southern periphery of the 100 kt jet. Also, 200 mb diffluence occurred over eastern New Mexico and western Texas, along with speed divergence between AMA and OKC. 43

2.4 20 October 1983 On 20 October, the remnants of Hurricane Tico were located over north-central Mexico, having just crossed the Sierra Madre Occidental mountain range. The circulation of Tico was still well-defined despite traveling over rugged terrain. By the day's end, the circulation center entered southwest Texas. Along with the direct influence of Tico, the synoptic conditons continued to provide the mechanisms for the heavy precipitation occurring over Texas and Oklahoma. Cumulative rainfall totals during the 48-hr period 19-20 October ranged from 7.3 inches over the South Plains (LBB) to near 10 inches in south-central Oklahoma (OKC).

2.4.1 20 October 0000 GMT Figures 2.13 a., b., c, and 2.14 a., b show the synoptic patterns at the surface, 850 mb, 700 mb, 500 mb, and 200 mb, respectively at 20 October 0000 GMT. The surface low pressure center near AMA 12-hours earlier moved eastward and was located northeast of OKC, with the cold front extending southwestward to southwest Texas. The warm front, which at the 17 October 0000 GMT was a stationary front over central Texas, stretched from the low eastward across northern Oklahoma and Arkansas. In general, the cyclonic circulation around the low pressure center became much better defined. South of the cold front, conditions had not changed greatly. Southeasterly winds (10-15 kt) continued to advect an abundant supply of Gulf moisture into the region. Temperatures remained in the upper 70'F 44

Figure 2.13. a) Surface, b) 850 mb, andc) 700 mb synoptic analysis at 0000 GMT 20 October 1983. Striped areas represent low-level moisture (surface dew point i 60'F (16'C) and 850 mb dew point i 15'C). 45

Fiqure 2 14. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 20 October 1983. Striped area represents 500 mb moisture (dew point i -10'C). 46 to low 80'F, with dew points in the upper 60'F to low 70'F. North of the frontal boundary, moderate north-northeast winds (10-25 kt) dominated most of the South Plains and Panhandle region, indicating strong convergence along the frontal zone. Much lower temperatures were located north of the front, with readings in the upper 40'F in the Panhandle to the mid 50'F over the South Plains. Nearly saturated condtions were observed over this region, with dew points ranging from the mid 40'F to the mid 50'F. At 0000 GMT, Lubbock reported heavy rain, with a temperature and dew point of 57'F (14'C) and 56'F (13'C), respectively. Winds were from the north at 10 kt, and the rising surface pressure was 1009 mb. During the previous 12-hours, heavy rain continued from LBB to OKC, as the surface cold front moved slowly southeastward. In the 12-hour period ending at 20 October 0000 GMT, LBB received over four inches of precipitation. The only severe weather event during the time period of this study, an F2 which touched down near Elgin, Oklahoma, occurred at 0000 GMT.

The broad area of low pressure at 850 mb over the southwest showed very slight intensification, while remaining quasi-stationary. Twelve-hour height falls of 10-30 m were observed throughout the area. Southeasterly flow at this level was confined to the southern half of Texas, with Gulf moisture abundant over this area. Colder air, out of the Northern Plains, entered the Texas Panhandle under the influence of strong, northerly flow (35 kt wind at AMA). Two major developments took place as a result. First, due to the northerly flow, 850 mb convergence developed from the 47

South Plains to north-central Texas. This combined with the existing surface convergence along the cold front, enhancing thunderstorm activity In the area. Second, the cold-air advectlon resulted In the development of a strong temperature gradient (baroclinic zone) in the heavy precipitation region. The temperature at AMA was 5'C, while at Stephenville (SEP) the temperature was 18'C. This temperature gradient remained strong over the next 24 hours.

The 700 mb trough and low pressure center continued to travel slowly towards the northeast, with no evidence of intensification. At this time, a short wave was moving through Texas and Oklahoma. Twelve-hour height falls of 20 m at AMA and MAF were an indication of the short wave. Southwesterly flow (20-35 kt) was common throughout the three-state region, with the exception of AMA, which reported a southeast wind at 10 kt. This resulted in strong 700 mb diffluence between AMA and OKC. Moisture from both the Gulf of Mexico and Pacific Ocean continued to flow into the region, with increasing moisture advection from the southwest. The trough and low pressure center at 500 mb weakened slightly and shifted to the north-northeast. No significant height changes were observed at this level over the region. Strong southwesterlies (30-40 kt) continued over the three-state area, with no evidence of diffluence or speed divergence. Also, there was no longer evidence of veering with height over north Texas and Oklahoma. Mid-level warm air advection from the southwest was still present, although the northern extent of the -5'C isotherm did not increase from its position 12 hours earlier. Areal 48 coverage of the mid-level moisture from the Pacific expanded significantly. The -10'C Isodrosotherm extended northward into southern Kansas, and eastward into southwestern Missouri and northwestern Arkansas. Heavy rains in Missouri during this study commenced, coincident with the arrival of abundant, mid-level moisture.

The polar jet stream has begun to slowly shift to the northeast. The 70 kt band extended from near ELP to New England. The 90 kt band stretched from south-central New Mexico to the Mid-Atlantic states. Extremely strong winds (greater than 100 kt) were still common from the southern Rockies to the Great Lakes, with 110 kt winds continuing over the Midwest. After landfall, the translational speed of the remnants of Hurricane Tico, through Mexico and eventually Texas, increased significantly under the influence of this strong polar jet.

2.4.2 20 October 1200 GMT Figures 2.15 a., b., c, and 2.16 a., b show the synoptic patterns at the five pressure levels at 1200 GMT. The remnants of Hurricane Tico were located near 5JT. While entering the highly baroclinic zone, the cyclone began to take on extratropical characteristics. Warm and cold frontal boundaries were associated with the low pressure center. The cold front stretched southwestward into Mexico, while the warm front extended into southern Missouri. A well-defined circulation was still evident jven though the storm had traveled over 2000 km since landfall. Throughout west Texas and Oklahoma, north winds of 10-25 kt fed cool damp air into 49

Figure 2.15. a) Surface, b) 850 mb, andc) 700 mb synoptic analysis at 1200 GMT 20 October 1983. striped areas represent low-level moisture (surface dew point i 60'F (16'C) and 850 mb dew point l 10'C). Figure 2.16. a) 500 synoptic analysis, and b) 200 mb isotach analysis at 1200 GMT 20 October 1983. Striped area represents 500 mb moisture (dew point i -10'C).

\ the now . Temperatures and dew points over west Texas were now in the low to mid 40'F, while a majority of Oklahoma had readings In the mid 50'F. At 1200 GMT, LBB experienced light to moderate rain with a temperature and dew point of 46'F (8'C), northerly winds of 25 kt, and a rising surface pressure of 1013 mb. During the previous 12-hours, heavy precipitation continued from the South Plains to northeastern Oklahoma. The largest amounts were recorded mainly in Oklahoma (four inches at OKC) along the warm frontal boundary, consistent with overrunning. Rainfall intensities decreased over the LBB area, although measurable amounts were still being recorded.

At 850 mb, a closed low (Tico) was located northwest of DRT, with no significant height changes in the three-state area. The low pressure center was west of the surface, or towards the colder air. This indicated that the vertical structure of Tico had begun to take on extratropical characteristics. Strong, south-southeasterly flow (30-40 kt) was widespread in central and south Texas, while strong, northerly flow (40 kt at AMA) occurred over the Texas Panhandle. Thus, strong 850 mb convergence continued in the heavy rain region. Cold air advected southward into eastern New Mexico and the South Plains , strengthening the temperature gradient over north-central Texas (2'C at AMA, 16'C at SEP). Heavy precipitation fell along this strong temperature gradient. The low-level moisture supply remained high, with the 850 mb moisture tongue (10'C isodrosotherm) extending well up into central Illinois. Greater 52 moisture concentrations (15'C) persisted in central and eastern Texas, as well as eastern Oklahoma. An elongated area of low pressure, stretching from northern Mexico to the Central Plains, dominated the 700 mb pattern. Two low pressure centers, one over eastern New Mexico (Tico) and the other in western Iowa, were evident, but no significant height changes were recorded. The low associated with Tico continued to tilt with height towards colder air. Moderate south-southwesterly flow (20-35 kt) continued to advect relatively warm, moist air into eastern Texas and Oklahoma. Colder air, in association with the circulation around the low pressure center over eastern New Mexico, began to slowly filter southward into west Texas. However, no extreme temperature gradient developed, as was the case at 850 mb. The maximum in moisture at this level shifted slightly eastward, correlating well with the observed eastward shift in the heaviest precipitation amounts. The closed low at 500 mb was located on the Arizona-New Mexico border, having traveled northeastward from its previous position. This low pressure center was now associated with the surface extratropical cyclone (Tico) located in Texas. The storm continued its tilt in the vertical towards the colder air, which was situated west of the pressure center, as the remnants of Hurricane Tico displayed extratropical characteristics. Twelve-hour height falls of 10-20 m suggested that the low would continue to travel east-northeast, parallel to the hurricane track. South-southwesterly flow of 30-50 kt continued to bring mid-level 53 tropical moisture into Texas and Oklahoma, with the moist tongue (-10'C isodrosotherm) now extending to western Tennessee. At this time, upper level diffluence was occurring over northeastern Oklahoma, where heavy rain fell. The 500 mb temperature pattern revealed some changes from 000 GMT. Mid-level warm air advection from the southwest, present during the past 24 hours, ceased. A pool of cold air, west of the 500 mb low, was located over eastern Arizona and western New Mexico. This cold air was beginning to advect into west Texas, contributing to the destabilization of the atmosphere in the layer below 500 mb, and the continued, widespread occurrence of rainshowers in this region. The main core of the polar jet stream decreased in size and shifted towards the northeast over the previous 12-hours. The band of 90 kt winds extended from the Central Plains to the Mid-Atlantic states, with the 110 kt jet shifting slightly eastward over Indiana and Ohio. The 200 mb winds over the region of heavy rain weakened and shifted to southwesterly at 50-70 kt. Diffluence was occurring at this time over the eastern portions of the Texas Panhandle and western Oklahoma.

2.5 21 October 1983 On 21 October, former Northeast Pacific Hurricane Tico, now an extratropical cyclone, entered Oklahoma and Missouri, having traveled through Texas. During the previous 48 hours, heavy rain fell from LBB to OKC, with amounts of nearly 10 inches measured in some locations. On this 54 day, rain continued in Oklahoma, with the largest amounts observed over the northeastern portion of the state. Heavy precipitation also occurred in Missouri and Illinois as the cyclone continued to travel towards the northeast.

2.5.1 21 October 0000 GMT Figures 2.17 a., b., c, and 2.18 a., b show the synoptic patterns at the surface, 850 mb, 700 mb, 500 mb, and 200 mb, respectively, at 21 October 0000 GMT when the surface low (Tico) was located near OKC. A slight decrease in its intensity took place over the previous 12 hours as its central pressure rose from 1004mb to 1009 mb. A well-defined cyclonic circulation was still evident as the storm moved northeastward. Moderate to heavy precipitation continued near the low pressure center and along the warm front, which extended northeastward into the Ohio Valley. During the previous 12-hours, rainfall in excess of 3 inches was observed near Tulsa (TUL), northeast of the low pressure center. In west Texas, cool, damp conditions and northerly winds persisted. Temperatures and dew points over the area were generally below 10'C. Light, rainshower activity continued in the South Plains, with moderate precipitation observed over the Texas Panhandle. Thunderstorm activity also occurred in south Texas, along the cold front which extended southward from the cyclone center. At 0000 GMT, LBB reported cloudy skies with light drizzle, while the temperature and dew point were 49'F (9'C) and 47'F (8'C), respectively. 55

dS pointT60-F (16'C) and 850 mb dew point HOC). 56

Figure 2.18. a) 500 mb synoptic analysis, and b) 200 mb isotach analysis at 0000 GMT 21 October 1983. Striped area represents 500 mb moisture (dew point i -10'C). 57

Winds were out of the north-northwest at 15 kt, with a rising surface pressure was 1016 mb. At 850 mb, the low pressure center that was near DRT 12-hours earlier was located near Wichita Falls (SP5), southwest of the surface low and tilted towards the colder air. There were no significant height changes observed at this time. Cyclonic circulation was now recognizable at the 850 mb level, in association with the low pressure center. Cold-air advection continued over the South Plains, while the strong temperature gradient remained in the region (4'C at AMA, 14'C at OKC, and 16'C at SEP). Strong, low-level moisture and warm-air advection occurred east of the low in Arkansas, Missouri, and Illinois, where most of the heavyier rain was then recorded. The 10'C isodrosotherm extended well into central Illinois. Southerly winds of 20-30 kt over this area advected the low-level, Gulf moisture northward. West of the low in west Texas, north-northwesterly winds of 20-30 kt were being observed the 850 mb convergence zone in Texas and Oklahoma, however, was no longer dominant as it had been during the previous 24-36 hours. At this time, a 700 mb closed low was located near AMA. There was also continued evidence of the storm center tilting with height towards the west (colder air). A height fall of 20 m was recorded at OKC, an indication that the low pressure center would continue to travel northeastward. Moderate southwesterly flow of 25-40 kt occurred from central Texas to southern Illinois, accompanied by warm-air advection, most notably from OKC (10'C) to Salem, Illinois (SLO, 4'C). Over west Texas, near the low 58 pressure center, winds were light and variable, with some cold air advectlon. Moisture advectlon at this level occurred from Monett, MO (UMN) to SLO. Nearly saturated conditions were also present in a band from ABQ to Peoria, Illinois (PIA). At 500 mb, the closed low was positioned over eastern New Mexico, near Tucumcari. Twelve-hour height falls of 30-40 m were observed in the Texas Panhandle (AMA) and Oklahoma, suggesting future eastward progression of the low pressure center. Moderate to strong southwesterlies of 30-50 kt were observed from Texas to Illinois, with warm-air advection taking place from SEP to UMN. Also, 500 mb diffluence was evident over northeastern Oklahoma. At this time, a pocket of cold air (-15'C) was located over the Texas Panhandle, with slight warm air advection from the west. Very dry air was now present at 500 mb throughout the three-state region, with the only signs of significant mid-level moisture stretching from central Kansas to near St. Louis, where the majority of the heavy precipitation occurred at this time. The main core of the polar jet stream continued to shift towards the east-northeast, while the band of 70 kt winds extended to far southwest Texas near ELP. The 90 kt isotach stretched from the upper Great Lakes to the East coast, while 110 kt winds were confined to the Mid-Atlantic states. This shifting of the main core of the jet has caused a 10-15 kt decrease in the forward speed of the extratropical cyclone. 59

2.5.2 21 October 1200 GMT Figures 2.19 a., b., c, and 2.20 a., b show the synoptic patterns at the five pressure levels 21 October 1200 GMT. The surface extratropical cyclone was located near UMN. The low pressure center continued to weaken slightly, with a central pressure of 1011 mb. The cold frontal boundary extended southward from the low through Arkansas and Louisiana, and along the Texas Gulf coast. The warm front stretched eastward through the Ohio Valley. A majority of the light to moderate precipitation occurred along the warm front in southern Illinois and Indiana. Light drizzle continued over Oklahoma and into the Texas Panhandle, while precipitation had stopped in the South Plains. High pressure in New Mexico began to influence the weather over west Texas. The cloud cover which had dominated the area for nearly a week rapidly dissipated, as clear skies spread over eastern New Mexico and far west Texas. Temperatures remained low, with readings in the mid to upper 40'F. At 1200 GMT, LBB reported partly cloudy skies, with a temperature of 48'F (9'C) and a dew point of 45'F (7'C). Winds were northwest at 10 kt, and the rising surface pressure was 1018 mb. The closed low at 850 mb continued to northeastward and was located northeast of OKC, near the Oklahoma/Kansas border. The low pressure center showed no signs of intensification. Twelve-hour height falls of 20-30 m were observed over Illinois and Missouri, an indication of further movement towards the northeast. Moderate to strong southerly winds (30-35 kt) continued east of the low along the Texas coast, and into 60

Figure 2.19. a) Surface, b) 850 mb, and c) 700 mb synoptic analysis at 1200 GMT 21 October 1983. Striped areas represent low-level moisture (surface dew point i60'F (16'C) and 850 mb dew point i 10'C). 61

Figure 2.20. a) 500 synoptic analysis, and b) 200 mb isotach analysis at 1200 GMT 21 October 1983. 62

Arkansas and Missouri. This low-level jet advected of warm, moist air as far north as central Illinois. In the meantime, north-northwesterly flow continued to advect lower temperatures Into west Texas and Oklahoma. The strong temperature gradient over central Texas remained, slightly shifting to the east. High pressure over the Rocky Mountain states has begun to drift over west Texas. The 700 mb closed low was located over south-central Kansas, near the Oklahoma Panhandle, with no signs of intensification. Twelve-hour height falls of 20 m In Kansas and Missouri suggested continued east-northeast movement. South-southwesterly winds of 25-30 kt advected warmer air into the Missouri Valley, while a pool of colder air settled over the Texas Panhandle region where northwesterly flow persisted. At this time, the only sign of significant 700 mb moisture occurred from the Ohio Valley to the Great Lakes. High pressure over the southwestern states built into the South Plains during the next 24-48 hours. At this time, the 500 mb closed low was centered over southwest Kansas. The vertical structure of the once tropical cyclone continued to show the extratropical characteristics of tilting with height towards the colder air, but no intensification. Twelve-hour height falls of 30-40 m were being observed over Kansas and Missouri, suggesting further movement to the east-northeast. A 500 mb jet of 40-45 kt stretched from southwest Texas to the Great Lakes, advecting very dry air into a once saturated region, along with warmer air. Colder air at this level was confined to the Rockies where northerly flow prevailed. The only sign of 63 significant mid-level moisture was in a narrow band from central Illinois to North Carolina. High pressure over the western half of the U.S., evident In the lower levels, was not so well-defined at 500 mb. The polar jet stream was concentrated from the Great Lakes to the East coast. The 200 mb Isotach pattern showed mainly 50-60 kt winds over the region of heavy rain. The main core of the jet (100 kt winds) was located from Pennsylvania to New Jersey. This persistent shifting of the jet stream caused a continued decrease in the speed of the low pressure system. Over the next several days, the surface extratropical cyclone, which was once Northeast Pacific Hurricane Tico, continued to travel towards the northeast, while also continuing to decrease in intensity. Significant precipitation accompanied the storm into the Great Lakes region, although it was not nearly as heavy as the rainfall amounts measured from Texas to Missouri. Residents living in this region were left to recover from the $93 million in damages which resulted from devastating flooding that the heavy rain event caused. Chapter 111 RESULTS AND ANALYSIS

This chapter presents a more detailed analysis of the heavy rain event that took place over the South Plains of Texas as the remnants of Northeast Pacific Hurricane Tico moved through the area. The first section examines the heavy precipitation that fell from LBB to OKC while the latter sections focus on the physical mechanisms and energy sources that led to the event.

3.1 Precipitation Analysis 18-21 October A three-day precipitation analysis, utilizing data from 81 recording stations in Texas and Oklahoma, was performed at 12-hour intervals in order to determine the manner in which the heavy rainfall occurred. Figure 3.1 shows the rain gauge network used in this analysis. Most of the precipitation took place during this time period, predominantly between 18 October 1200 GMT and 21 October 0000 GMT. The heavy rain event actually began during the evening hours on 16 October and early morning hours on 17 October, as thunderstorms with radar echo tops of 47 kft (1 kft= 1000 feet) traveled through the Texas Panhandle and South Plains (Fig. 3.2.a). These storms merged with other cells in northeastern Oklahoma to form a line stretching from near Carlsbad, NM, to St. Louis (Fig. 3.2.b). Brief, heavy rainfall occurred with these storms, although their rapid speed (20-25 kt) prevented excessive amounts. By 1200 GMT, the precipitation had ended

64 AM ' * •

LBQ : • ^P^

'mf

Figure 3.1. The Texas-Oklahoma rain gauge network. 66

T'\ '\L/ / .1;

MONDAY ^ 13SZ d£V 17. '983 RAi^AR. 1

MONDAY M35Z dt'V 17." 1983 RW^Afl.

Figure 3.2. Radar summary at a) 0135 GMT, b) 0435, GMT 17 October 1983. 67 over the South Plains. The next 24-36-hour remained relatively rain-free, until the core of the heavy rain event commenced. In the time period from 0000 GMT to 1200 GMT 18 October rainfall was very sparse over west Texas. An Isolated thunderstorm with a maximum top of 43 kft, was located over the MAF-BIg Spring (BGS) area of the Permian Basin (Fig. 3.3.a). BGS received 1.4 Inches of rain from this storm as It moved eastward at 15 kt. This cell developed north of the quasi-stationary frontal boundary (Fig. 2.7a). Figure 3.3.b shows the 12-hour observed rainfall over the Texas-Oklahoma network during this time period. At 1435 GMT 18 October, a broad area of relatively light thundershowers was located over a large portion of central Oklahoma, with the strongest cell (41 kft or 41,000 feet) just north of SPS (Fig. 3.4.a). Also, scattered rainshowers developed over extreme southwest Texas. Visible satellite imagery from GOES-West at 1515 GMT shows extensive cloud cover over west Texas and Oklahoma (Fig. 3.4.b). The cloud cover extended southwest to the northeast Pacific Ocean where the northern portion of Hurricane Tico could be seen off Baja California. By 2035 GMT, a large area of showers and thunderstorms had developed over eastern New Mexico and far west Texas (Fig. 3.5.a). These storms, with tops as high as 45 kft, were moving north-northeastward at 40 kt. Showers were also located in northeastern Oklahoma. During the next 3 hours, the showers and thunderstorms had entered west Texas, stretching from the Texas Panhandle near AMA to the Permian Basin, south of MAF (Fig. 3.5.b). Movement was more the easterly at 25 kt as the cells remained strong. 68

-Nt Nt

IS- TUESDAY • HOS OtT 18. 1983 RAl^AR,

Figure 3.3. a) Radar summary at 0135 GMT 18 October, b) 0000 GMT to 1200 GMT 18 October observed rainfall (inches) 69

I.f .»

7^ _ ^^ TUESDAY ^ .. ISZ.OCjr 18. 19.83 RAi^Ap.

Figure 3.4. a) Radar summary at 1435 GMT 18 October, b) visible imagery at 1515 GMT 18 October. 70

2V TUESDAY ^ . Q^ 18. 1983 RAi^A^.

|2U

Nt INE ^\3i\/ es*a.

tU-iI

' TUESDAY ot*V 18. 1983 RWDAP

Figure 3.5. Radar summary at a) 2035 GMT andb) 2335 GMT 18 October. 71

Showers were also forming over far southwest Texas near ELP. Precipitation during the past 24-hour exceeded 1 Inch near Plainview, with over one-half inch measured throughout the Texas Panhandle. Also, a band of rainfall oriented west-east of at least 0.5 Inch was observed in central Oklahoma, with 1.1 inches measured at OKC and 1.5 Inches in eastern Oklahoma. Figure 3.6 shows the 24-hour (0000 GMT 18 October to 0000 GMT 19 October) observed precipitation over the Texas-Oklahoma network.

During the next 12-hours (0000 GMT to 1200 GMT 19 October), heavy rainfall, with amounts exceeding 4 inches, took place from the South Plains to south-central Oklahoma, as the warm front moved northward through the area, while the cold front approached from the northwest (Fig. 2.9a). This occurred as Hurricane Tico approached the west coast of Mexico. At 0235 GMT 19 October, a band of showers and thunderstorms oriented southwest-northeast could be seen stretching from west-central Oklahoma near GAG to ELP (Fig. 3.7.a). Cells in this line of storms reached as high as 39 kft (39,000 feet) south of Wink, TX (INK), as they traveled northeastward at 15-20 kt. Enhanced IR imagery from GOES-E at 0300 GMT showed the line of convective activity, coinciding with the 0235 GMT radar summary (Fig. 3.7.b). A majority of this activity was in the medium gray (-32'C to -41'O and light gray (-41'C to -52'C) scales, indicating of fairly cold cloud tops. There were embedded regions of heavier showers as revealed by the dark gray (-52'C to -58'C) scale. This scale was an indication of higher and colder cloud tops, or deeper convection. Towards the southwest, more abundant and intense showers could be seen extending to the Mexican west coast, and out to sea. This enhanced IR imagery was Figure 3.6. Observed rainfall (Inches) from 0000 GMT 18 October to 0000 GMT 19 October. 73

ft V.' 2U ©"""^

IJEDNESDAY ^ , OCT 19. 19.83 RAijAR.

Figure 3.7. a) Radar summary at 0235 GMT 19 October b) enhanced infrared imagery at 0300 GMT 19 October. 74 associated with Hurricane Tico, which was within 12-hours of landfall. Enhanced Infrared imagery at 0600 GMT showed that the area of dark gray enhancement over north-central Texas and south-central Oklahoma increased In size (Fig. 3.8.a). Small areas of black enhancement (-58'C to -62'C) were also now being detected. Off the coast of Mexico, repeat gray and white enhancement (-62'C to -80'C and colder), in association with Tico, could be seen near the coastline. At 0900 GMT, the region of black enhancement (-58'C to -62'C) in south-central Oklahoma also increased in size (Fig. 3.8.b). This intense convection was now located southwest of OKC. Meanwhile, medium (-32'C to -41 'O and light gray (-41 'C to -52'C) enhancement covered a large portion of the state. A new area of heavy showers (-52'C to -62'C cloud top temperatures) had developed over west Texas, north of MAF, with more developing in southwest Texas. The large area of repeat gray and white enhancement (Tico) continued to travel towards the northeast. Tico was then within 6-hour of landfall. The radar summary at 1135 GMT showed a long line of showers and thunderstorms stretching from southwest Texas all the way to the Great Lakes region (Fig. 3.9). Cells, with tops of 38 kft (38,000 feet), were located south of OKC, coincident with the infrared black enhancement there. An embedded, more intense storm was located near MAF. This storm had cloud tops of 47 kft (47,000 feet) and was associated with the dark gray (-52'C to -58'C) and black (-58'C to -62'C) enhancement in that region. Shallower clouds (26 kft tops or 26,000 feet) covered a large portion of the South Plains. This broad area of precipitation traveling northeastward at 15 kt, ensuring continued rainfall for the South Plains region including LBB. 75

'^:^^

Fiaure 3 8 a) Enhanced infrared imagery at a) 0600 GMT and b) 0900 GMT 19 October 76

STC nu

I SIC) su NE 25 .111 NE NE- "•" 2f^ \ TW«

NE u IRU» 2ZC 2s: NE \

WV, ln« O^

TMC^ ei^1C1l.«tM . -^iiEnNf^nAV '1 \\35i0cr{%\^d^

Figure 3.9. Radar summary at 1135 GMT 19 October. 77

During the previous 12-hour, rainfall exceeding 1 inch occurred from the Permian Basin near Andrews northeastward to the OKC area. Heavier amounts of precipitation were measured along the Texas-Oklahoma border and into south-central Oklahoma. Local amounts of up to 4 Inches were reported near Lawton, OK; while, well over 2 inches of rainfall were observed over a large portion of north-central Texas and south-central Oklahoma. Figure 3.10.a shows the 12-hour (0000 GMT to 1200 GMT 19 October) measured precipitation over the Texas-Oklahoma network. Cumulative rainfall totals from 0000 GMT 18 October to 1200 GMT 19 October showed that the LBB area has received over 2 inches of rain, with larger amounts measured towards the east-northeast. Lubbock International Airport (LBB) received 2.03 inches of precipitation during this time period. Figure 3. lO.b shows the cumulative rainfall over the Texas-Oklahoma network.

From 1200 GMT 19 October to 0000 GMT 20 October very heavy rainfall (4 inches) took place over central portions of the South Plains, as the strong cold front pushed through the area (Fig. 2.1 la). During this time, Hurricane Tico made landfall and continued to travel towards the northeastward, entering southwest Texas by the end of the 12-hour period. Visible satellite imagery from GOES-W taken at 1431 GMT 19 October showed extensive cloud cover from the west coast of Mexico, northeastward through Texas and Oklahoma, and into the upper Midwest region (Fig 3.1 l.a). Hurricane Tico could be seen making landfall, with the eye still quite noticeable. The radar summary at 1435 GMT indicated a broad area of relatively light rain and thundershowers stretching from Figure 3.10. Observed rainfall (inches) from a) 0000 GMT to 1200 GMT 19 October and b) 0000 GMT 18 October to 1200 GMT 19 October i «PI1

pyw.

\ NEI- \. NE \ ^..^ • -

^^ TJEDNESDAY ^ •U35Z dtiV 19." 1983 RAI^AR

Figure 3.11. a) Visible imagery at 1431 GMT 19 October andb) Radar Summary 1435 GMT 19 October. 80 southwest Texas to northeast Oklahoma (Fig. 3.1 l.b). Echo tops over the South Plains were generally in the area of 25 kft (25,000 feet). Highest tops (33 kft or 33,000 feet) were located over south-central Oklahoma. All activity moved north-northeast at 15-20 kt. At 1735 GMT, thundershowers continued over the entire South Plains region and a large portion of central Oklahoma (Fig. 3.12.a). Tops continued to be in the range of 20-25 kft. An embedded more Intense cell (35 kft or 35,000 feet) developed southwest of LBB and moved over the area within the next two hours. Movement northeastward at 10-20 kt.

At 2031 GMT, visible imagery continued to show extensive cloud cover over eastern New Mexico, western Texas, and Oklahoma (Fig. 3.12.b). Also, the circulation of Tico could be detected over central Mexico despite its having crossed the Sierra Madre Occidental mountain range. Enhanced infrared imagery one-half hour later (2101 GMT) showed an area of light (-41 'C to -52'C) and dark gray (-52'C to -58'C) enhancement over the southern sections of the South Plains and northern sections of the Permian Basin. Also, medium and light gray enhancement covered much of the Texas-Oklahoma Panhandle region (Fig. 3.13.a). Over central Mexico, a large area of dark gray and black (-58'C to -62'C) enhancement was located in the region of the circulation center. The radar summary at 2135 GMT showed thundershowers covering all of the Texas Panhandle and South Plains regions, along with the Permian Basin and much of the state of Oklahoma (Fig. 3.13.b). Echo tops remained in the area of 20-25 kft (20,000-25,000 feet), with a small area of more intense showers (35 kft or 35,000 feet) located southwest of LBB and a larger area in central 81

^\ \E-^ J N£/^^:

IdEDNESDAY \ ; fSS oev )9: 19^3 RA9*«

Figure 3.12. a) Radar summary at 1735 GMT 19 October and b) visible imagery at 2031 GMT 19 October. V 7 y 3:

>-ai.._ • \ __,-^-^ TJEDNESDAY V • /QO ^^rp,.».^\--*''1?n5Z. OCT 19. 19.83 RAI^AR.

fMT^iS^n^A^'^- ^l Enhanced infrared imagery at 2101 Sober. ' ' ^^ ^^^^' '^^^'^ ^^ ^' ^5 GMT 19 83

Oklahoma (32kft or 32,000 feet). All echoes continued to travel towards the northeast at around 10 kt. Enhanced Infrared imagery (Fig. 3.14) taken at 2303 GMT was consistent with the 2135 radar summary, showing a large area of light gray (-41 'C to -52'C) enhancement over the Texas Panhandle and South Plains regions. Also, an area of dark gray (-52'C to -58'C) enhancement located south of LBB matched the 35 kft (35,000 feet) cell in that region. An area of dark gray (-52'C to -58'C) and black (-58'C to -62'C) enhancement was still situated the remnants of the circulation center of Tico. Over the previous 12 hours, rainfall exceeding 3 inches was observed from the South Plains to central Oklahoma. Local amounts of over 4 inches were also reported. LBB officially received 4.31 inches of precipitation during this period, while Brownf ield, Texas (southwest of LBB) received 4.2 inches. Figure 3.1 S.a shows the 12-hour (1200 GMT 19 October to 0000 GMT 20 October) observed precipitation over the Texas-Oklahoma network. Cumulative rainfall totals (48 hr), shown in Fig. 3. IS.b, exceeded 6 inches both over the LBB vicinity and in west-central Oklahoma (Southwest of OKC). A band of 3-inch rainfall extended from the extreme southern sections of the South Plains to east-central Oklahoma. From 0000 GMT to 1200 GMT 20 October, heavy rainfall continued over a large portion of central Oklahoma, with amounts greater than 4 inches measured near OKC. Moderate rain continued over the South Plains, as another inch accumulated. The remnants of Hurricane Tico, now an extratropical cyclone, were located over central Texas near SJT by the end of the time period, still moving northeastward. The warm front, extending 8^

^K.-r'^JH^ '^•^'^- Enhanced infrared imagery at 2303 GMT 19 October. ^ ^ 85

Figure 3.15. Observed rainfall (inches) from a) 1200 SMT 19 October to 0000 GMT 20 October and b) 0000 GMT 18 October to 0000 GMT 20 October. 86 northeastward from the cyclone center into Oklahoma (Fig. 2. IS.a) was the focal point for the heavy precipitation. Enhanced infrared Imagery at 0001 GMT 20 October showed a large area of light gray enhancement (-41 'C to -52'C) over the Texas Panhandle, South Plains, and western Oklahoma (Fig. 3.16.a). A broken line of dark gray enhancement (-52'C to -58'C) stretched from LBB to MAF, indicating deeper clouds. The area of thundershowers from the Texas-Mexico border southwestward was associated with the remnants of Tico. The radar summary at 0135 GMT showed a large band of generally light to moderate precipitation over west Texas and Oklahoma (Fig. 3.16.b). Radar echo tops were only around 20 kft (20,000 feet); however, there were two regions of stronger thunderstorms, one area east of MAF (44 kft or 44,000 feet) and the other area southwest of OKC (45 kft or 45,000 feet). The latter caused the F2 tornado near Lawton. Examination of radar imagery at this time revealed that the tornado occurred when convection was organized in isolated cells, one reaching 45kft (Byrd and McCaul, 1985). Enhanced infrared at 0300 GMT showed the area of dark gray enhancement (-52'C to -58'C) moving off to the east of LBB, while areas of medium and light gray (-41 'C to -52'C) enhancement covered much of the Panhandle region (Fig.3.17.a). The area of white enhancement (below -80'C) near DRT (the remnants of Tico) indicated the re-intensification of the storm as it became extratropical. The radar summary at 0435 GMT did not reveal any significant changes from 0135 GMT 20 October (Fig.3.17.b). Light thundershowers (20 kft or 20,000 feet) still covered a large portion of west Texas and Oklahoma, with the heaviest occurring south of OKC (40 kft m;

11

THURSDAY ' \ V 135Z.QI!J" 20.' 19.83 R«J«^.

Fiqure 3.16. a) Enhanced infrared imagery at 0001 GMT 20 October and b) radar summary at 0135 GMT 20 October. ^ THURSDAY ' \ •.• M3SZ o'c'V 20. 19.83 RApAR i m-i \ 1 »••'

Figure 3.17. a) Ehanced infrared imagery at 0300 GMT 20 October and b) radar summary 0435 GMT 20 October. 89 or 40,000 feet). The precipitation area was starting to break up over eastern New Mexico and western Texas. This trend continued over the next several hours. Also, notice the deeper showers from the Concho Valley (SJT) to the Texas-Mexico border (42-48 kft or 42,000-48,000 feet). They were associated with the white enhancement (below -80'C), which was located near the cyclone center at 0300 GMT. All the showers continued to travel towards the northeast.

At 0600 GMT, enhanced infrared imagery showed thundershowers intensified over the previous three hours, mainly over central Oklahoma (Fig. 3.1 S.a). At this time, a broken line of intense thundershowers (dark gray (-52'C to -58'C) and black (-58'C to -62'C) extended from DRT northeastward to near OKC. An area of white enhancement was still located near the storm center. By 0900 GMT, infrared imagery continued to show strong activity in Oklahoma, while the activity near the remnants of Tico, although still intense, no longer displayed white enhancement (Fig. 3.18.b). The radar summary at 1135 GMT continued to show thundershowers over a large portion of the Texas Panhandle and Oklahoma (Fig. 3.19). Also, precipitation continued throughout central Texas, as the showers left the South Plains and Permian Basin. Over the previous 12 hours (Fig. 3.20.a), OKC received 4.63 inches of rainfall, with two other rain-gauge stations in central Oklahoma measuring over 4 inches. A band of 2-inch rainfall extended from the extreme eastern portions of the South Plains to northeastern Oklahoma. LBB received 1.22 inches of precipitation during the 12-hour time period. Cumulative precipitation from 0000 GMT 18 October (Fig. 3.20.b) exceeded 6 inches over the central portions of the Figure 3.18. Enhanced infrared imagery at a) 0600 GMT and b) 0900 GMT 20 October. 91

..^r^ \ • ^^^^^ IHJRSOAY

Figure 3.19. Radar summary at 1135 GMT 20 October. 92

Figure 3.20. Observed rainfall (Inches) from a) 0000 GMT to 1200 GMT 20 October and b) 0000 GMT 18 October to 1200 GMT 20 October. 93

South Plains (LBB), and southwestern Oklahoma (OKC). Rainfall exceeding 2 inches stretched from the Texas-New Mexico border to extreme northeastern Oklahoma. Most of the measurable precipitation that occurred during the next 12-hours was confined to Oklahoma, as the activity over the South Plains ceased. Heaviest amounts of rainfall (3 Inches) were recorded near TUL, while over I inch was observed throughout central Oklahoma. During this time, the extratropical cyclone (Tico) continued moving northeastward. By 0000 GMT 21 October, the cyclone center was located near OKC (Fig. 2.17.a). Enhanced infrared Imagery at 1200 GMT showed that the only area of heavy showers (dark gray (-52'C to -58'C) and black (-58'C to -62'C) enhancement) was located over central Texas near the cyclone center (Fig. 3.2l.a) with lighter showers from central Oklahoma to the Panhandle. The radar summary at 1435 GMT showed weak thundershowers (24 kft or 24,000 feet) throughout Oklahoma, while stronger storms were located in central Texas (Fig.3.21.b). All showers continued to travel northeastward, as precipitation rates rapidly decreased over the South Plains region. Visible imagery at 1830 GMT showed a large comma-shaped cloud mass over Texas and Oklahoma (Fig. 3.22.a). The center of the extratropical cyclone (Tico) was located near SPS. By 2035 GMT, only lingering rainshowers in the Texas Panhandle remained, while the majority of the rainshowers were over Oklahoma (Fig. 3.22.b). Thunderstorms also developed along the cold front which extended southward from the low pressure center. 3"

^^,,*^ THURSDAY \ ^i 435Z.0"cJr 20." 19.83 RAI^Ap. rs^-r'^oSHC^.-^-^^- ^^ Enhanced infrared imagery at 1200 GMT 20 October and b) Radar summary at 1435 GMT 20 October. 95

aa' Ksr. ^^^ • \ . • "

^ THURSDAY ^ [Q^ 20." 19.83 RAI^AR.

Figure 3.22. a) Visible imagery at 1835 GMT 20 October and b) Radar summary at 2035 GMT 20 October. 96

The greatest amounts of precipitation over the previous 12-hours in north-central Oklahoma where in excess of 3 inches were measured (Fig. 3.23.a). OKC reported 2.69 Inches of rainfall during this time. Cumulative (72 hr) precipitation (Fig. 3.23.b) exceeded 11 Inches for central Oklahoma (11.7 Inches at OKC), while the South Plains (LBB) received rainfall in excess of 7 inches.

Figures 3.24.a and 3.24.b show the cumulative precipitation for LBB and OKC, respectively. Notice that the two displayed similar patterns, although OKC received nearly 4 inches more than LBB. The heavy rain event at both cities began with relatively light rain for about 12-24-hours. The heaviest rainfall at LBB took place in a 24-hour time period (0000 GMT 19 October to 0000 GMT 20 October), while the most intense rainfall at OKC began 12- hours later and lasted for 24-36 hours. An interesting result was the 12-hour lag between the times of the heaviest precipitation at LBB and OKC. Over the next 12 hours, precipitation ended over the flood stricken areas of Texas and Oklahoma. Radar summaries at 0135 GMT and 1135 GMT 21 October showed showers moving to the northeast into Missouri and Illinois (Figs. 3.25.a, b). This movement coincided with the continued northeastward path of the extratropical cyclone.

3.2 Relative Humiditv Analvsis 17-21 October This section examines these two parameters as Tico traveled northeastward through Texas and Oklahoma. Figures 3.26.a-e shows the patterns of relative humidity/precipitable water versus time for DRT, SEP, 97

Figure 3.23. Observed rainfall (Inches) from a) 1200 GMT 20 October to 0000 GMT 21 October and b) 0000 GMT 18 October to 0000 GMT 21 October. 98 98

Inches 12

10 LBB

8

6

4

2

18/00 18/12 19/00 19/12 20/00 20/12 21/00

Date/Tfme (GMT)

18/00 18/12 19/00 19/12 20/00 20/12 21/00

Dote/Time (GMT)

Figure 3.24. Cumulative rainfall (Inches) from 0000 GMT 18 October to 0000 GMT 21 October for a) Lubbock (LBB) and b) Oklahoma City (OKC). 99

2V FRIDAY ^ . OCT 21. 1983 RADAR

FRIDAY V V OCT 21. 1983 RADAR

Figure 3.25. Radar summary at a) 0135 GMT andb) 1135 GMT 21 October. 100

17/00 18/00 I9/0O 20/00 21/00 17/00 18/00 19/00 20/00 21/00

o«ti/TtiiM

100 LSI 1.46 90 nAF

00

RH 70

60

SO

40 009 0.37 0.50 H 1-

I7/0O 10/00 19/00 20/00 21/00 17/00 10/00 19/00 20/00 21/00

0«t«/TlnM (CnT) Da(t/Tlm« (CKT)

too

1.34 90 IS4 OKC

00

RH 70

60 1.14

SO 069 40

17/00 10/00 19/00 20/00 21/00

0«t*/Tlm« (cm)

Figure 3.26. Surface-500 mb average relative humidity (%)/ precipitable water (inches) versus time for a) DRT, b) SEP, c) MAF, d) AMA, and e) OKC. ' A ' ' ' I v./ I I I

MAF, AMA, and OKC, respectively. The atmosphere over Texas and Oklahoma became increasingly moist as Hurricane Tico approached the Mexican coastline, illustrated by increases in both the average relative humidity (sfc-500 mb) and precipitable water over the region, during the time in which the heavy rains occurred. Conditions at the beginning of the analysis (0000 GMT 17 October) were relatively dry throughout Texas and Oklahoma. Surface- 500 mb average relative humidities ranged from 43% at DRT and MAF, to 62% at OKC and 65% at AMA. Precipitable water ranged from .83 inches (171% of normal) at AMA, to 1.14 inches (163% of normal) at OKC. During the next 48-72 hours, steady increases took place in both the relative humidity and precipitable water, most notably at DRT, SEP, and MAF. This increase coincided with the beginning of the core of the heavy rain event over the South Plains. Between 19-20 October, sfc-500 mb relative humidities reached as high as 88% at DRT, 92% at SEP, and 95% at MAF. Precipitable water amounts reached as high as 1.86 inches (200% of normal) at DRT, 1.80 inches (216%) at SEP, and 1.51 inches (255%) at MAF. By 20 October the circulation of Tico was located over central Texas. At OKC, between 0000 GMT 17 October and 0000 GMT 19 October, an oscillation in the relative humidity/precipitable water patterns occurred. Maximum values in both parameters were at 1200 GMT, while minimum values were at 0000 GMT. Between 0000 GMT 19 October and 1200 GMT 20 October, a general increase in both parameters took place at OKC. The sfc-500 mb relative humidity reached 88% by 1200 GMT 20 October, while the amount of precipitable water increased to 1.54 in (228%). This 102 increase at OKC coincided with the core of the heavy rains that took place in that area. AMA showed similar pattern. After 1200 GMT 20 October, a steady decrease in both the sfc-500 mb average relative humidity and precipitable water occurred at all of the stations. This decrease coincided with the end of the heavy rain event.

3.3 Instabilitv Analvsis 17-21 October This section will examine two commonly used stability indices (K-lndex and Lifted Index) at the upper-air stations in Texas and Oklahoma from 17-21 October (Appendix B). Figures 3.27.a-e show the patterns of KI/LI versus time for DRT, SEP, MAF, AMA, and OKC, respectively. The atmosphere over Texas and Oklahoma steadily destabilized prior to and during the heavy rain event. Significant increases in the K-lndex (Kl) took place between 17-20 October at DRT and MAF, with a tripling in the value of Kl at DRT during this period. This increase was mainly due to the increase of moisture at the 850 mb, 700 mb, and 500 mb levels. Values of the lifted index (LI) also revealed a fairly unstable atmosphere over DRT (LI of -5), and MAF (LI of -3) as the heavy rains began over the South Plains. At OKC, an increase in Kl was noticed during the time when the heavy rains began in this region. Values of LI, however, did not reveal this destabilization, probably because OKC was situated north of the quasi-stationary frontal boundary and experienced lower surface temperatures than DRT and MAF throughout the entire heavy rain event. The same is true for AMA, as the values of Kl and LI did not indicate any significant destablization. By 21 October, both indices revealed substantial stabilization over Texas and Oklahoma 103

40

DRT 30 -- 30 -- -3

Kl Kl

20 -- 20 --

10 10

a 17/00 18/00 19/00 20/00 21/00 IJ 17/00 10/00 19/00 20/00 21/00

0«tt/TlnM

40

ruF 30 -- 30 --

Kl Kl

20 20 --

10 10

17/00 10/00 19/00 20/00 21/00 17/00 10/00 19/00 20/00 21/00

0«te/TlnM

OKC

30

Kl

20 --

10

e 17/00 10/00 19/00 20/00 21/00

Oata/TIm* (GMT)

Figure 3.27. K-lndex/Lifted Index versus time for a) DRT, b) SEP, c) MAF, d) AMA, and e) OKC. 104

Similar analyses of moisture and instability were performed on an earlier heavy rain event that occurred in Texas and Oklahoma as Northeast Pacific Hurricane Norma (1981) traveled through the region (Johnson and Mortimer, 1983), with similar results. Substantial increases in the sfc-500 mb average relative humidity and precipitable water were noted prior to and during the heavy rain event, along with a destabilization in the area. The amount of precipitation recorded in the October 1981 heavy rain event (Norma) exceeded rainfall amounts measured in this study.

3.4 Objective Analvsis of Mixino Ratio 19-20 October An objective analysis scheme was employed to analyze important meteorological parameters such as mixing ratio, horizontal divergence, surface moisture convergence, and absolute vorticity. The scheme consisted of a 15 x 25 rectangular grid system from which grid points were calculated using the 43 rawinsonde stations. Figure 3.28 shows the grid system/rawinsonde stations used in the objective analysis. This section will examine the objective analyses of the mixing ratio fields (g/kg) at four levels (sfc, 850, 700, 500 mb). The following sections will examine horizontal divergence, surface moisture convergence, and absolute vorticity, along with vertical motion and streamline analyses. Figures 3.29a.-d. show the objective analysis of mixing ratio at the surface 850, 700, 500 mb. At 0000 GMT 19 October, surface mixing ratios up to 20 g/kg extended northwestward to the South Plains; while at 850 mb the 16 g/kg mixing ratio isopleth was draped over the area. Lower mixing 105

Figure 3.28. Grid system and upper air stations used in objective analysis. 106

HIKING RATIO I C / KC I eOZ 19 OCT 1983 MIXIMG RATIO I C / KC I eez 19 OCT 1983 MM://' V

'~~ 1 "^ ,' " ^^^"'^V^

/ A^ /v

^ 729.0 \ ^ _-^ •% "*•

INTERVAL. 2.0 SUHfACE b INTERVALm. 2. 6 esa na

HIKING RATIO I C / KG I eez 19 OCT 1983 MIXING RATIO I G / KG eez 19 OCT 1983

INTERVAL. 2.e 7ee ns INTERVAL. 2. a see na

Figure 3.29. Mixing ratio (g/k^ objective analyses for 0000 GMT 19 October at a) surface, b) 850 mb, c) 700 mb, and d) 500 mb. 107 ratio values can be seen over the northwestern portions of the grid system, extending southeastward over northern Mexico. Moisture from the Pacific Ocean seemed to be a contributing factor at the 850 mb level. Tropical moisture from the Pacific was apparent at both 700 and 500 mb. The 10 g/kg mixing ratio isopleth can be seen extending to the southern sections of the Texas Panhandle at 700 mb; while at 500 mb the 4 g/kg isopleth extended northeastward over the area . The moisture content at these levels would increase over the next 12-24 hours. Figures 3.30a.-d show objective analysis of mixing ratio at the surface 850, 700, 500 mb. At 1200 GMT 19 October, the surface and 850 mb moist axis took on a more negative tilt (NW-SE orientation). At the surface, the 20 g/kg isopleth now extended to the southern Panhandle region, and into eastern New Mexico. At 850 mb, the 18 g/kg isopleth now reached the South Plains, with the 20 g/kg isopleth reaching the Permian Basin. The Gulf of Mexico now was the dominating source for the moisture at 850 mb. At both lower-levels a strong moisture gradient was present over eastern New Mexico, as was the case during the previous 12-hours. Mid-level, tropical moisture increased significantly over western Texas, especially at 500 mb. At 700 mb the 10 g/kg isopleth continued to stretch to the northern South Plains; while larger moisture amounts (12g/kg) were located over southwest Texas. Much drier air was confined to central and eastern Texas. A much larger increase in moisture occurred at 500 mb as the 6 g/kg isopleth now extended up to the South Plains. The previous 12-hours found this moisture confined to central Mexico. As 108

MIXING RATIO ( G / KG I 12Z 19 OCT 1983 HIKING RATIO I C / KG I 12Z 19 OCT 1983 .llLiyTZim 'C 1 + Vj y y^

\ \ \ L lLta»e^ jP \ 1 '*'^\\V\ \l 1 / K. • \ A. \. 1 \

INTERVAL. 2.6 SURFACE INTERVAL. 2.0 esa MB

HIKING RATIO I G / KG I 12Z 19 OCT 1983 MIXING RATIO ( C / KG 12Z19 OCT 1983

INTERVAL. 2.0 700 MB U INTERVAL- 2.e see H8

Figure 3.30. Mixing ratio objective analyses for 1200 GMT 19 October at a) surface, b) 850mb, c) 700 mb and d) 500 mb. 109

earlier stated, this substantial increase in 500 mb moisture coincided well with the period of heaviest rainfall over the South Plains. Figures 3.3la.-d show objective analysis of mixing ratio at the surface, 850 mb, 700 mb, and 500 mb. Slightly drier, post-frontal air from the northwest began to enter the Texas Panhandle and South Plains region at the lower-levels by 0000 GMT 20 October. The northern extent of the 20 g/kg Isopleth at the surface was now from M/U^ to OKC. Recall that at this time Oklahoma began to receive its heaviest rainfall amounts. Moisture at 850 mb also began to shift towards the east, as the surface cold front continued to propagate in this direction. The 16 g/kg isopleth was now confined from the extreme southern sections of the South Plains, stretching up through eastern Oklahoma. While moisture at the lower-levels began to decrease, mid-level moisture concentrations continued to increase across western Texas and Oklahoma. At 700 mb, the 12 g/kg isopleth now extended up to the Texas Panhandle; while the 10 g/kg isopleth stretched all the way up to central Missouri, a significant increase from the previous 12-hour position. Drier air continued to be located over eastern Texas. Sufficient amounts of moisture at 500 mb continued to be present. The 6 g/kg isopleth enclosed the entire western half of Texas; while 4 g/kg mixing ratio values reached the state of Illinois. Over the next 12-hours this mid-level moisture would also begin to shift east-northeastward as the source of the moisture (Tico) traveled towards the northeast. Low-level moisture continued to shift eastward by 1200 GMT 20 October, as the larger concentrations were now confined to eastern Texas 110

HtXINC RATIO ( C / KC I •«Z 2« OCT 1963 HUIMC RATIO I C / KC I e«Z 3« OCT 1983 ^UHt-?^

2^.^\ j < I 10.1

\ / INTERVAL. 2.0 SUWACC IMTCRVAL. 2.0 r-1 \8S0 lm

HIXIMC RATIO I C / KC 1 e«Z ?« OCT 1983 MIXING RATIO 1 C / KC 1 eez 2« OCT 1903 */ 1 4 < 1 I* > \ 1 1 /

MIXING RATIO ( G / KG I 12Z 20 OCT 1983 HIKING RATIO I C / KC 1 12Z 20 OCT 1983

a INTERVAL. 2.0 SURFACE t) INTERVAL. 2.0 850 MB

HIKING RATIO I C / KC ) 12Z 20 OCT 1983 MIXING RATIO ( C / KC I 12Z 20 OCT 1<)e3

Q INTERVAL. 2.e 7ee MB d I'^TERVAL. 2.0 see MB Figure 3.32. Mixing ratio (g/kg) objective analyses for 1200 GMT 20 October at a) surface, b) 850 mb, c) 700mbandd) SOOmb. 113

3.5 Objective Analvslqnf Divergence 19-20 October

This section will examine horizontal divergence (x E-5 1/sec'M at seven levels (sfc, 850, 700, 500, 400, 300, and 200 mb), in order to determine If patterns favorable for the development of upward vertical motion were present In the areas where the heaviest rainfall took place. Figures 3.33a.-c show objective analysis of horizontal divergence at surface, 850, and 700 mb. At 0000 GMT 19 October, surface convergence was evident from central New Mexico (ABQ), throughout a large portion of Texas, and Into central Oklahoma near OKC. This area of horizontal convergence was in response to the NW-SE oriented warm frontal boundary, which was located over the region at this time. A local convergence maximum (divergence minimum) was situated over west Texas, in the vicinity of LBB. This correlated well with the location of intense convective activity that was present throughout west Texas during this time period. LBB reported a thunderstorm at 0000 GMT 19 October. At 850 mb, a large area of horizontal convergence was again present over eastern New Mexico, western Texas, and southwestern Oklahoma, with a local maximum located southwest of LBB. Convergence also appeared throughout Texas and Oklahoma at 700 mb, although no local maximum was present over the South Plains at this level. Figures 3.34a.-d show objective analysis of horizontal divergence. While convergence was prevalent throughout the lower-levels, horizontal divergence dominated the upper-levels at 0000 GMT, especially at 300 mb. At 500 mb, slight divergence was present throughout Texas, except for the southwestern sections of the South Plains and the Panhandle region. At 114 }

OtvCRCCNCC I • ie-S / SEC I 002 1« OCT 1903 DtVCRCCNCE I • IC-S / SCC I 00Z 19 OCT 1903 jL^ / <32 ^-^JA"^ 1 1.V '^'f \ 4- j Z"^"^'

J^\ >._:tA^C1 \ ^y ^^ ^\<-:i I * 7\ -IST INTERVAL. 3.0 SURTACt INTERVAL. 2.0 es0 M8

OI«CI«CCMCC • • tC-« / SCC 1 MI «4 OCT ««•!

1 J 1 ^N * •'^I'.^N. •* V 3IAI T7 VH^^ \ 1J44 ,."

(•TtRVM.. 3.0 7M ng

Figure 3.33. Horizontal divergence (10"- sec" ^) objective analyses for 0000 GMT 19 Oct at a) surface, b) 850 mb, and c) 700 mb. 1 c; I -I

OIVCRCCMCC 1 • 1C-S / KC 1 MX 19 OCT «««3 ei«C«GCMCC I • 1C-4 / see • MZ 14 OCT 1903 * \(1^-'. 1 0 • 1 V

Vi^- (^ iHUevM.. 3.0 l«TCJIV«L. 3.0

OlvCMCCHCC I • 1C-1 / UC I MI 19 OCT 1M3 OIVCMCCNCt I . 1C-f / KC I MI 19 OCT IM]

V'. ^r*. 'r1 * ^ * i' *tv \'• "'k\ "Z* J1^ --' -'' J

f ^ ^^^^ r LN^V / * /

•^-^* *\ / "A

^ 2l^

l«T(M««(.. 3.e IHTtRVM.. 3.e 3M na Figure 3.34. Horizontal divergence (10"^ sec" ^) objective analyses for 0000 GMT 19 October at a) 500 mb, b) 400 mb, c) 300 mb and d) 200 mb 116

400 mb, this area of divergence extended northward, covering much of Oklahoma and the eastern Texas Panhandle, with slight convergence positioned over western sections of the Panhandle and South Plains. An area of relatively strong horizontal divergence, stretching from the Gulf of California to central Oklahoma, appeared at 300 mb. Two distinct areas of maximum divergence were located over western Texas, one near ELP and the other over the Panhandle and South Plains region. Strong 300 mb diffluence over New Mexico, Texas, and Oklahoma resulted in this area of horizontal divergence. Also playing a role was the speed divergence that was occurring between CUU and MAF, and between AMA and OKC at this time. The areas in which lower-level convergence and upper-level divergence were taking place correlated well with the regions where heavy precipitation was presently occurring. Figures 3.35a.-c show objective analysis of horizontal divergence at surface, 850, and 700 mb. By 1200 GMT 19 October, the low-level convergence that had become established over the previous 12 hours remained intact. Surface convergence continued over the western half of Texas, and a large portion of Oklahoma. This was in conjunction with the frontal boundaries that were draped across the region at this time. This region of horizontal convergence was located in an area where an abundant supply of moisture was already in place, and increasing. The area of surface convergence also correlated well with where the heavy precipitation was currently taking place. An interesting feature was the good correlation between the zero divergence line and the southern extent 117

•191MCHC C I • 1C-« / IK 1 tax 19 0cr i9n I • ic<« / «e I «n f« 0CT 1901 V \\^r _1^\ f • 1 • -1^3 14. i 4. /\ **^"

***>_^>^i ^S W * •V >• T N A /

\ v.. ir ^i v ) \..i» ittmvM.. 3.0 IMiaVAL. 3.0

OIVCnCCNCC < • 1C-« / KC I 131 19 OCT 1903

IHTtHVAL. 3.0 ?M M0

Figure 3.35. Horizontal divergence (10"^ sec" ^) objective analyses for 1200 GMT 19 October at a) surface, b) 850 mb and c) 700 mb. sSsi'i'

118 Of the precipitation (radar) echo return. Much stronger convergence was present at both 850 and 700 mb over large portions of Texas and Oklahoma. At both levels, particularly 850 mb, areas of maximum convergence were located between LBB and OKC. This again correlated well with the region of heavy rainfall. Also, the area of strong convergence over western Mexico (MZT) was a reflection of Hurricane Tico, which at this time was making landfall over the region.

Figures 3.36a.-d show objective analysis of horizontal divergence at 500, 400, 300, and 200 mb. Strong, upper-level divergence continued over large portions of Texas, New Mexico, and Oklahoma at 1200 GMT. This was especially the case at the 400, 300, and 200 mb levels. At 400 mb, two areas of maximum divergence were present, one over eastern New Mexico, and the other over north-central Texas; while at 300 mb even stronger divergence was located over eastern New Mexico and much of southwest Texas. A region of strong, 200 mb divergence was also located over the Panhandle and eastern South Plains region. Strong, upper-level diffluence, downstream of the west coast, long-wave trough, was the main reason for the horizontal divergence appearing over these regions. As was the case during the previous 12 hours, the areas of divergence occurred over areas where lower-level convergence was taking place. This pattern would imply that upward vertical motion continued throughout the region, and again correlated well with the regions of heavy precipitation and large moisture concentrations. Recall that the South Plains (LBB) received its largest rainfall amounts during this time period. Section 3.8 will further examine the vertical motion fields at 500 mb throughout Texas, Oklahoma, and New 1 i9 5

eivCMCCMCC « • lC-9 / KC • laZ 1« OCT 1««3 eiVCNCOCC I • 1C-9 / KC I 131 19 OCT I993

1 L''"""""'-!* »'*"* • V 1 h-— ,, I "'"•- ' —>1.9» I \*1 ^__L**^*^^ 4-•9/" N M \ T»>v^ fi--*:—

'*>•". k ! lanWAL. 3.0 l«TCRV«L. 3.0

oinaccMcc I • «c-t / UC I 131 t« OCT (Ml OlvCMCCHCC I • 1C-4 / KC 1 171 1« OCT «M3

L Wii r ' 1 It -1. '7 • VJ^\ J /"NT ift4

1 M iK • / l.ai

•/ * 1 1^ ^ *\ \i. 1 J\ I M \

*J \ \ ^i.ii^^ ^s^,.^ r^\ \> V"C >'* ^\^ ^. -

InTtltVM.- 3.0 }«• >« IHTCRVM.. 3.0 )M ««

Figure 3.35. Horizontal divergence (10"^ sec" ^) objective analyses for 1200 GMT 19 October at a) 500 mb, b) 400 mb, c) 300 mb and d) 200 mb. i20

Mexico. This fields will be computed using the kinematic method which Involves the integration of horizontal divergence field up to 500 mb. Figures 3.37a.-d show objective analysis of horizontal divergence at 500, 400, 300, and 200mb. Strong, low-level convergence, especially 850 mb, remained the dominant feature over much of west Texas at 0000 GMT 20 October. At this time, a strong, SW-NE oriented cold front extended from a surface low over north-central Oklahoma towards the southwest to near the Big Bend region of Texas. At the surface, a region of maximum convergence was located east of LBB; while at 850 mb a large area of very strong convergence covered much of the South Plains and Permian Basin regions. Moderate precipitation was occurring over all of west Texas and Oklahoma at this time, correlating well with these areas of low-level convergence. A region of heavier activity over the Concho Valley was located just south of the areas of strongest convergence. Also the activity south of OKC, which produced the lone severe weather event, did not correlate with any regions of maximum convergence. Another interesting feature was how the western edge of the precipitation region over western Texas and eastern New Mexico coincided with the zero divergence line at the surface. Figures 3.38a.-d show objective analysis of horizontal divergence at surface, 850 and 700 mb. Weak convergence was located over the area at 700, and 500 mb at 0000 GMT, with divergence occurring over southwest Texas. Widespread upper-level divergence continued to cover the entire area at this time, with the strongest divergence occurring at the 300, and 200 mb levels. Strong upper-level diffluence was again the main 121

•IVEnCENCE 1 • 16-5 / SEC 1 MZ 3« OCT I«e3 OIVERCENCE I • 1E-5 / SEC I •«I 30 OCT IMS w / / "S '1 S) * f / L / •" • *; 1. ^ \JP * -'•"/Jt^"^ Vu '• / — y{ /* 4 1 * .ar+ r / H / r^'^'s^* I .^^r;^^\^^

* \ /—s. 'V^^ v* ^^«^

f ^"^ \. \ -^1" INTCRVAL. 3.* INTERVAL. 3.0 SURr*CE est na

OIVERCENCE 1 • IE •i / SEC 1 MI 2« OCT 1983 / 1' / ,S*H r •».

* > * • •f 1 + \ + /"^

(L f^^"*-^,*'j.5 4 ' .SA*

1.64 1

^r\ V J ^ ' • \ \_^ \«-*\ \, 7 \ yv )*'•' ^^ t aie / INTERVAL. 3.0 7ee MS

Figure 3.37. Horizontal divergence (10 ^ sec M objective analyses for 0000 20 October at a) surface, b) 850 mb andc) 700 mb. 122

oivenccNCE i • ic-« / KC I MI 30 OCT IMS oivcRGENce I • ie-s / KC I eez 30 OCT IMS i 1 ( J w \ $-f* fv 4 1 * \\ ^ r^ "£^»•! Y~-i • ^^K *\ L^ ^] r^y, • INTEnvM,. 3.0 «M m INTCRVM.. 3.0

OlvCRCENCe I • 1C-9 / KC 003 30 OCT 1903 OIVERCCNCC I • ie-S / KC I MZ 30 KT 1M3 V-^ ' 1 1 LV • 1 \ M A^ )M^"/ / "•t^fc^^^ • 1 • 1 4 ^)*v ^ 1 \ * •L.»^W ,^ I'll \

.2\r v^ -y fi IHTCRVM.. 3.0 300 r« INTCRVM.. 3.0 3M N0

Figure 3.38. Horizontal divergence (10"^sec-1) objective analyses for 0000 GMT 20 October at a) 500 mb. b) 400 mb, c) 300 mb and d) 200 mb. 123 contributor. The areas of divergence at these levels occurred over regions of strong lower-level convergence, as was the case during the previous 12 hours. This pattern again implied the presence of upward vertical motion In an area where an abundant supply of moisture was available (Fig.3.31a-d). This combination resulted in the large rainfall totals recorded from LBB to OKC during this time. Figures 3.39a.-c shows objective analysis of horizontal divergence at the surface, 850 and 700 mb. At 1200 GMT 20 October, low-level convergence continued over a large portion of Texas and Oklahoma, although weaker than the previous 12 hours. At this time, the extratropical cyclone (Tico) was located over central Texas. This was reflected by the convergence maxima located over this region, at both the surface and 850 mb. Precipitation, which had been decreasing over west and southwest Texas was now confined to central Texas, along the cold front, and to the Texas Panhandle, along with much of Oklahoma. These regions of rainfall were located north-northwest of the warm frontal boundary. In general, the areas of rainfall continued to correlate well with the regions of low-level convergence. Again, a correlation could be made between the zero divergence line and the northern fringe of the precipitation region, especially from the Texas Panhandle, extending northeastward. At 500 mb, relatively strong convergence was also occurring over the Panhandle, and up into Colorado and Kansas. This along with the lower-level divergence in the area prohibited any significant rainfall totals from taking place. Figures 3.40a.-d show objective analysis of horizontal divergence at 500, 400, 300 and 200 mb. Upper-level (400 mb) divergence was present I 4LH

OIVERCENCE 1 . ie-5 / SEC 1 I3Z 30 OCT IM3 OIVERCENCE I • IE-S / SEC I 13Z 3* OCT IMS ••• J L -1 .91 T \j »^ 1 1.6 \y^ * 1 4 /~ 1 \l t -(T '4 -1

'N •'-"i ^\ + ^\\ .000 \ 7/^ -.47e INTERVM.. 3.0 SURFACE INTCRVM.. 3.0 850 MS

OIVERCENCE I • IE-S / SEC I I3Z 30 OCT 1M3

INTCRVM.. 3.0 700 no

Figure 3.39. Horizontal divergence (10^ sec" ^) objective analyses for 1200 GMT ^0 October at a) surface, b) 850 mb, and c) 700 mb. 125

OIVERCENCE I . 1C-S / UC 13Z 30 OCT IMS OIVERCENCE I • 1C-S / UC I I3Z 30 OCT IMS ~I "^ n ,^. / f—\ '"• r'^^t * U' N N \ / " 1 . 1 4 \ 1.05 *\ •¥ '•2| + 1 4 /

F^*v^^ -^ Ky , V 4I ) ^)A*^~~\ ' X '^ ^V3^ V-1.54 •( N \ .670 \ ^V?;-'. -'• + ^^ INTCRVM.. 3.0 SOO no INTCRVM.. 3.0 400 no

OIVERCENCE I > IE-S / UC I I3Z 30 OCT l«03 OIVERCENCE I • IE-S / UC I I3Z 30 OCT IMS

:NTCRVM.. 3.0 300 m INTCRVM.. 3.0 3M no

Figure 3.40. Horizontal divergence (10^ sec" ^) objective analyses for 1200 GMT zO October at a) 500 mb, b) 400 mb, c) 300 mb and d) 200 mb. 126 over northern Texas and Oklahoma; while at 300 and 200 mb, divergence dominated the entire two-state region at 1200 GMT. Maximum regions of divergence were located over southwestern Oklahoma at both levels. Again, the areas of significant precipitation correlated well with the regions where lower-level convergence and upper-level divergence occurred, namely over northern Texas and Oklahoma, where the bulk of the moisture was now located (Fig. 3.32). Vertical profiles of horizontal divergence were calculated at MAF, AMA, and OKC for the time 19-20 October. The profiles were analyzed using the mandatory levels from the surface to 100 mb. Again, these were examined In order to determine if patterns favorable for the development of upward vertical motion were present in the regions of heavy rainfall. Figures 3.41 a.-d show the vertical profiles of horizontal divergence for MAF. At 0000 GMT 19 October, maximum convergence (-1.79 E-5 sec" ^) could be seen at 850 mb. Divergence would gradually increase with height, up to a maximum (1.60) at 300 mb. The level of zero-divergence was approximately 550 mb. Twelve hours later (1200 GMT), low-level convergence would continue, although slightly weaker. Maximum convergence (-1.20) again occurred at 850 mb. A gradual increase in horizontal divergence again took place between 850 mb 400 mb, with the zero-divergence level rising to 500 mb. A sharp increase took place between 400 mb and 300 mb, as the upper-level divergence reached a maximum (2.29) at the 300 mb level. By 0000 GMT 20 October, the general pattern of the previous 24-hours had changed. Lower-level convergence increased significantly as the 127

100- I riAF 200- 200 MAF

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o<.o-^ .-" 0(10-* ,-"

Figure 3.41. Vertical profiles of horizontal divergence (10"^ sec"') for MAF at a) 0000 GMT, b) 1200 GMT 19 October, c) 0000 GMT and d) 1200 GMT 20 October. 128 maximum (-3.79 E-5 sec"l) continued to be at 850 mb. A rapid increase in divergence occurred between 850 mb and 700 mb, with slight convergence again present at 500 mb. Another increase took place between 500 mb and 300 mb, as maximum divergence (1.85) again was located at the 300 mb level. At 1200 GMT, low-level convergence continued, with the 850 mb maximum (-1.98) somewhat weaker than the previous 12 hours. A general increasing divergence pattern took place between 850 mb and 200 mb, with maximum divergence (1.53) now located at this level. This general pattern of low-convergence and upper-level divergence was conducive to the development of upward vertical motion over this region. Figures 3.42a.-d show the vertical profiles of horizontal divergence for AMA. At AMA, the main feature at 0000 GMT 19 October was strong upper-level divergence. A significant increase in divergence had occurred between 500 mb and 300 mb, with the maximum (2.82 E-5 sec"M located at this level. Below 500 mb, no strong features were present with only slight convergence evident down to 700 mb. By 1200 GMT, stronger low-level convergence was now present, especially at 850 mb, with the maximum (-1.82) located at this level. With decreasing pressure, horizontal divergence gradually increased to a maximum (2.03) at 200 mb. Strong divergence was also present at 400 mb. The large increase in low-level convergence coincided with the time the South Plains received its largest rainfall amounts. The strength of the low-level convergence at 850 mb weakened slightly by 0000 GMT 20 October. Also, the slight convergence that was present at 500 mb 12-hours earlier strengthened, and was now equal to the magnitude W:\

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Figure 3.42. Vertical profiles of horizontal ?o^/S^??5S®il' 9 sec-1) for AMA at a) 0000 GMT, b) 1200 GMT 19 October, c) 0000 GMT, and d) 1200 GMT 20 October. 13C Of the 850 mb convergence. Above 500 mb, divergence rapidly increased to a maximum (1.96) at 300 mb (Fig. 3.16). A large increase in divergence took place at the 300 mb level over the past 12 hours. The main feature at 1200 GMT was the even stronger convergence maximum (-2.78) at 500 mb. This along with the lack of lower-level convergence helped suppress any substantial upward vertical motion from continuing below the 500 mb level. A large increase in horizontal divergence occurred between 500 mb 200 mb, with the maximum (2.58) located at the 200 mb level.

Figures 3.43a.-d show the profiles of divergence at OKC for 19-20 October. At 0000 GMT, no significant features in the divergence cross-section were noted. Slight divergence was present at the low-levels (sfc, 850 mb) and above 400 mb, with slight convergence located between 800 mb and 500 mb. This pattern coincided with the lack of significant ' precipitation in the region at this time. By 1200 GMT, a divergence pattern more favorable to the development of upward vertical motion became apparent. Low-level convergence was now evident from the surface up to 500 mb, with the maximum (-1.37 E-5 sec"h located at 850 mb. Above 500 mb, horizontal divergence increased to a maximum (1.36) at 300 mb. Over the next 12-24 hours this pattern would continue, coinciding with the period of heaviest rainfall over Oklahoma.

At 0000 GMT, convergence extended from the surface up to 500 mb, with the maximum (-0.86) located at the 500 mb level. One would expect the strongest convergence to occur at the lower-levels during the time of heaviest rainfall; however, nearly equal convergence (-0.79) was present at 131

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Figure 3.43.-Vertical profiles of horizontal divergence (10"^ sec"') for OKC at a) 0000 GMT, b) 1200 GMT 19 October, c) 0000 GMT, and d) 1200 GMT 20 October. 132 the surface. Above 500 mb, divergence steadily Increased to a maximum (1.71) at 200 mb. Although the convergence (divergence) was not strong at the lower (upper)-levels, enough was present In order to induce the upward motion, hence heavy precipitation. Twelve hours later, both convergence and divergence at the respective levels strengthened. Convergence again was evident from the surface to the maximum (-1.54) at 500 mb. Convergence at 850 mb also increased over the past 12 hours. Much stronger upper-level divergence was present above 500 mb. A rapid increase took place above this level to a maximum (3.11) at 200 mb.

3.6 Objective Analysis of Moisture Convergence 19-20 October This section will examine surface moisture convergence (x E-5 g/kg sec) in order to determine whether excessive amounts of moisture were concentrated in regions where heavy amounts of precipitation took place. At 0000 GMT 19 October, a large area of strong moisture convergence (43.5) was centered over the South Plains near LBB (Fig. 3.44). This region was located southwest of the NW-SE oriented warm frontal boundary, and near the maximum convergence zone at this time (Fig. 3.33). Mixing ratios over this area were in excess of 20 g/kg (Fig. 3.29.a). At 2335 GMT 18 October, thunderstorms with cloud tops exceeding 40 kft were located from the Texas Panhandle to the Permian Basin (Fig. 3.5b), coinciding with the region of maximum surface moisture convergence. By 1200 GMT, maximum surface moisture convergence weakened slightly (40.4), and also shifted eastward (Fig. 3.45). Moisture convergence now M CONVERGENCE I « 1E-5 G/KG SEC I 00Z 1<) OCT 1983

INTERVAL. 6.0 SURFACE

Figure 3.44. Surface moisture convergence (10"^ g/kg sec) objective analysis for 0000 GMT 19 October. 134

M CONVERGENCE I K IE-5 G/KG SEC I 12Z 19 OCT 1983

INTERVAL. 6.0 SURFACE

Figure 3.45. Surface moisture convergence (10"^ g/kg sec) objective analysis for 1200 GMT 19 October 135 was covering much of northern and central Texas, with the maximum center near ABI. This region was southeast of the surface low located over the Texas Panhandle, and between the two frontal boundaries. Another region of very strong moisture convergence was located over central and western Mexico. This was associated with Hurricane Tico, which was making landfall at the time. Mixing ratio values Increased over Texas during the past 12 hours; however, the relatively strong surface convergence present over the South Plains at 0000 GMT weakened. This resulted in the slight decrease in the overall surface moisture convergence. At 1135 GMT, heavy rain and thunderstorms extended from southwest Texas to Oklahoma (Fig. 3.9). Again this correlated well with the regions dominated by moisture convergence. The most intense thunderstorm activity, over the Permian Basin, was situated southwest of the convergence maximum. Twelve hours later (0000 GMT 20 October) the maximum region of moisture convergence at the surface again strengthened (44.0), while continuing to shift eastward (Fig. 3.46). This area was located along the strong cold front that stretched from north-central Oklahoma to southwest Texas. A strong wind shift line associated with the front, along with mixing ratio values of around 20 g/kg, resulted in the strengthening of the moisture convergence. A strong moisture convergence gradient was also present over the Texas Panhandle and South Plains, behind the surface front. Moisture convergence could also be seen extending southward into central Mexico, where another maximum (48.0), associated with the remnants of Hurricane Tico, was located. At 0135 GMT, a large band of heavy rain was present over western Texas and Oklahoma (Fig. 3.16b). The 136 36

M CONVERGENCE ( x IE-5 G/KG SEC 00Z 20 OCT 1983

INTERVAL. 6.0 SURFACE

Figure 3.46. Surface moisture convergence (10 ^ g/kg sec) objective analysis for 0000 GMT 20 October. 137

core of the heavy rains stretched from the South Plains to central Oklahoma. This particular region was located along the strong surface moisture convergence gradient. In general, the area of rainfall coincided well with the regions of moisture convergence, with again the heaviest rains occurring along the strong gradient. By 1200 GMT, a band of relatively strong moisture convergence, although slightly weaker than the previous 12 hours, stretched from central Oklahoma to south of the Rio Grande River, with the maximum (38.8) located south of SPS (Fig. 3.47). This region fell along the frontal boundaries, which now extended from the extratropical cyclone (Tico), near SJT at this time. Moisture divergence now covered most of the Texas Panhandle and western South Plains, where drier air was also present. A relatively strong moisture convergence gradient still existed over southwestern Oklahoma. At 1135 GMT, the heaviest rainfall continued throughout Oklahoma and southward along the front to near DRT. This again correlated well with the surface moisture convergence zone. Lighter, dissipating rainfall lingered over the Panhandle where moisture divergence began to dominate. Precipitation gradually exited the South Plains.

3.7 Objective Analysis of Absolute Vorticitv 19-20 October This section will examine absolute vorticity ( x E-5 sec"b at 500 mb in" order to determine if the effect of positive vorticity advection (PVA) at this level contributed to the overall heavy rain event over Texas and Oklahoma. At 0000 GMT 19 October, the vorticity maximum (21.8), and associated 500 mb low, was located over Baja California (Fig. 3.48). Over 138

M CONVERGENCE I > 1E-5 G/KG SEC I 12Z 20 OCT 1983

INTERVAL. 6.0 SURFACE

Figure 3.47. Surface moisture convergence (10'^ g/kg sec) objective analysis for 1200 GMT 20 October. 139

ABSOLUTE VORTICITY I , IE-5 / SEC I 00Z 19 OCT 1983

INTERVAL. 2.0 500 MB

Figure 3.48. 500 mb absolute vorticity (10"^ sec"' )^objective analysis for 0000 GMT 19 October. 140

Texas and Oklahoma, an area of low absolute vorticity (minimum) was located near OKC, with slightly larger values to the west-southwest. A NW-SE oriented vorticity lobe (8.0) was also situated over northeastern New Mexico at this time. This minor upper-air disturbance aided in the development of the thunderstorms that took place over eastern New Mexico and western Texas during this time. The general flow pattern at the 500 mb level was from the southwest; therefore, the majority of the significant positive vorticity advectlon was presently occurring over southern Arizona and New Mexico, with the exception of the previously mentioned minor disturbance Cvort lobe"). Twelve hours later (1200 GMT), the vorticity maximum weakened slightly (18.8), while remaining stationary (Fig.3.49). The 500 mb low moved towards the east, now located over the Gulf of California. Much stronger vorticity was now present over New Mexico as the 10.0 ( x E-5 sec"M isopleth extended to the central portion of the state, with the 8.0 (x E-5 sec" ^) isopleth nearly reaching the Texas border. Southwesterly flow at 500 mb resulted in stronger positive vorticity advection over eastern New Mexico and the Texas Panhandle, with weaker positive vorticity advection occurring over the South Plains. This strengthening in the positive vorticity advection coincided with the period of heaviest rainfall over the South Plains. By 0000 GMT 20 October, the vorticity maximum over Baja California continued to weaken (13.3), and was now located over northern Mexico, near the position of the 500 mb low (Fig. 3.50). The vorticity over west Texas strengthened during the past 12 hours, as the low pressure system 141 141

ABSOLUTE VORTICITY I « lE-5 / SEC I 12Z 19 OCT 1983

INTERVAL. 2.0 500 MB

Figure 3.49. 500 mb absolute vorticity (10"^ sec"'fobjective analysis for 1200 GMT 19 October. 142

ABSOLUTE VORTICITY I •• IE-5 / SEC I 00Z 20 OCT 1983

INTERVAL. 2.0 500 MB

Figure 3.50. 500 mb absolute vorticity (10"^ sec"' f objective analysis for 0000 GMT 20 October. 143 approached from the southwest. The 8.0 ( x E-5 sec'^) Isopleth was now on a AMA-LBB-MAF line, while the 10.0 ( x E-5 sec"b Isopleth reached southern Colorado. Slight positive vorticity advectlon now was taking place over all of west Texas and western Oklahoma, as southwesterly flow persisted at the 500 mb level. This 500 mb positive vorticity advection gave upper-air support to the low-level moisture convergence occurring over Texas and Oklahoma, aiding in the development of the heavy rains that took place during this time. At 1200 GMT, the vorticity maximum (13.7) was now located over extreme southwestern Texas near ELP (Fig. 3.51). This maximum was east-southeast of the 500 mb low now positioned over the Arizona-New Mexico border. Vorticity continued to increase over the western half of Texas and Oklahoma. The 8.0 ( x E-5 sec"b isopleth was now on a SEP-DRT line, while the 10.0 ( x E-5"b isopleth reached the western South Plains. Another pocket of stronger vorticity (10.0) was located over western Oklahoma. Southwesterly flow at 500 mb continued the production of positive vorticity advection over Texas and Oklahoma at this time. This gave upper-air support to the surface convergence (frontal boundary) over central Oklahoma, where the majority of the heavy rainfall was continuing to take place. Although relatively strong 500 mb positive vorticity advection remained over the South Plains; the surface convergence and moisture were well to the east, thus preventing any further significant precipitation. ABSOLUTE VORTICITY I M IE-5 / SEC I 12Z 20 OCT 1983

INTERVAL. 2.0 500 MB

Figure 3.51. 500 mb absolute vorticity (10"^ sec"' f objective analysis for 1200 GMT 20 October. 145 3.8 Vertical Motion AnalygiQ 19-20 OctobeT — The kinematic method was used to calculate 500 mb vertical motions ( x E-3 mb/sec) over New Mexico, Texas, and Oklahoma during the heavy rain event. This analysis was performed subjectively, and Involved the Integration of the horizontal divergence field (Sect. 3.7) from the surface up to 500 mb. These fields were looked at in order to determine whether upward vertical motion was present in the areas of heaviest rainfall. At 0000 GMT 19 October, upward vertical motion (negative values) was present over New Mexico and Oklahoma, along with the western half of Texas (Fig. 3.52). The strength of the upward motion increased towards the west. The zero isopleth can be seen on a DRT-SEP line, extending northeastward into extreme southeastern Oklahoma. Descending air (positive values) was confined to the eastern half of Texas. The region of upward motion over western Texas correlated well with the regions of lower-level convergence/upper-level divergence (Fig. 3.33.a-3.34d). This also correlated well with the area of heaviest rainfall occurrence at this time (Fig. 3.5.b, 3.7.a). However, the relatively strong upward motion over extreme western Texas (ELP) did not correlate particularly well with the divergence patterns at that time; although, there was strong 300 mb divergence over the area, and thundershowers were present near ELP. Twelve hours later (1200 GMT), upward vertical motion continued to be located over Oklahoma, and a large portion of Texas (Fig. 3.53). The strongest region was now over central Oklahoma near OKC (-3.8 x E-3 mb/sec). The -2.0 isopleth encompassed the entire South Plains and Permian Basin regions. The major change in the vertical motion field, from 146

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Figure 3.52. 500 mb vertical motion (|ib/sec) analysis for 0000 GMT 19 October.

lb. 14

Figure 3.53. 500 mb vertical motion (ub/sec) analysis for 1200 GMT 19 October. 148 the previous 12-hours, was over extreme western Texas (ELP), where descending air was now occurring. Downward motion also continued over southeastern Texas.

The region of upward motion atl200 GMT correlated fairly well with the surface convergence pattern at this time (Fig.3.35.a). The zero vertical motion Isopleth was shifted slightly farther east than the zero divergence Isopleth. Recall that this comparison was subjective vs. objective in nature. Convergence was also present at the other lower-levels, while divergence occurred at the upper-levels in the areas of ascending motion. Again, a strong correlation could be made between the regions of upward motion, and where the heaviest precipitation was currently taking place (Fig.3.9), recalling that the South Plains (LBB) received its largest rainfall amounts during this time. In the areas of descending air over southeast Texas, little or no rainfall was detectable. At 0000 GMT 20 October, upward vertical motion was now covering the entire three-state region (Fig. 3.54). Only extreme southern Texas experienced descending air motion. The strength of the upward motion over northern Texas and southern Oklahoma increased over the past 12 hours. The -3.0 ( X E-3 mb/sec) isopleth now enclosed a large portion of the Texas South Plains and Panhandle, along with north-central Texas. Maximum upward motion was near AMA (-3.9). The -2.0 isopleth also expanded in areal coverage. The strengthening of the upward vertical motion over the South Plains corresponded to the strong surface, and 850 mb convergence present over the region at this time (Fig. 3.37a,b). Also, strong upper-level divergence, • fi '/ 149

Figure 3.54. 500 mb vertical motion (|ib/sec) analysis for 0000 GMT 20 October. 150 particularly at 300 mb (Fig. 3.38.c), aided in this strengthening. Extensive rainfall continued over all of west Texas and Oklahoma with central Oklahoma (OKC) receiving its largest amounts during this time. This precipitation pattern continued to correlate well with the areas of upward vertical motion.

By 1200 GMT 20 October, upward vertical motion was confined to the northern half of Texas and Oklahoma (Fig. 3.55). However, strengthening of the upward motion continued over west-central Oklahoma, and the eastern Panhandle regions. The -5.0 ( x E-3 mb/sec) isopleth now enclosed this region, with maximum vertical velocities over AMA (-5.6) and OKC (-5.2). Downward motion was now taking place over the southern half of Texas, with maximum descending air located over the extreme southern part of the state. The strong upward motion over Oklahoma and north Texas was in response to the low-level convergence, particularly at 850 mb (Fig. 3.39), and the strong upper-level divergence at 300 and 200 mb (Fig. 3.40.c,d). Widespread rainfall continued over the Texas Panhandle, and a large portion of Oklahoma (Fig. 3.19), with largest amounts during the next 12 hours occurring over central and northeastern Oklahoma. This correlated well with the areas of strong upward vertical motion. Although strong upward motion was present over the Texas Panhandle, moisture levels were low, thus preventing large rainfall amounts. Also, the precipitation over central and south-central Texas was occurring even though slight downward motion was present in this region. Note that in the area of maximum downward motion, little or no rainfall was detectable. 151

Figure 3.55. 500 mb vertical motion (|ib/sec) analysis for 1200 GMT 20 October.

In., 15o

3.9 Streamline Analysis 19-2QQcfnhPr This last section will examine the objectively analyzed streamline patterns from the surface up to 200 mb. This was performed in order to view the correlation between the general flow fields and the convergence/divergence patterns at the various levels. This analysis will also be used to examine the possible moisture sources at the seven levels. Figures 3.56 a-c and 3.57 a-d show the streamline patterns at the seven levels (sfc, 850, 700, 500, 400, 300, and 200 mb) for 0000 GMT 19 October. Two main features could be seen on the surface streamline pattern. First of all, strong, southeasterly flow from the Gulf of Mexico was occurring over a large portion of Texas. Secondly, a distinct convergence zone could be seen over the South Plains, south of the frontal boundary presently located over the northern Panhandle region. This zone was reflected by the maximum in the surface moisture convergence field over this region at this time (Fig. 3.44). Southeasterly flow was also apparent at the 850, and 700 mb levels. The general streamline flow pattern veered with height, with south-southwesterly winds occurring at the upper-levels (500, 400, 300, and 200 mb). The 500 mb long-wave trough could be seen over Baja California. Also of notice was the general anticyclonic curvature to the streamline pattern at these levels, particularly 200 mb. This curvature resulted in the horizontal divergence observed over Texas at this time (Fig. 3.34a-d). This upper-level flow pattern also enabled the advection of tropical moisture from the Pacific Ocean into the southwestern U.S. Figures 3.58a-c and 3.59a-d show the streamline patterns for the seven levels at 1200 GMT 19 October. Low-level (sfc, 850 mb), southeasterly 153

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Figure 3.57. Streamline objective analysis for 0000 GMT 19 October at a) 500 mb, b) 40(J mb, c) 300mbandd) 200 mb. 155

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7ae na Figure 3.58. Streamline objective analysis for 1200 (3MT 19 October at a) surface, b) 85(J mb, and c) 700 mb. 156

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//v\ /S 3aa Ma 2oe na Figure 3.59. Streamline objective analysis for 1200(3MT 19 October at a) 50(3 mb, b) 40(J0 mb, c) 300 mb and d) 200 mb. 157

flow continued the Influx of Gulf moisture into Texas and Oklahoma. The surface convergence zone, indicated by the streamlines over the South Plains 12 hours earlier, was no longer evident. This was reflected by the decrease in the surface moisture convergence over west Texas at this time (Fig. 3.46). The streamline flow pattern again veered with height. Upper-level flow continued from the south-southwest, as the anticyclonic curvature of the streamlines over Texas remained. This again correlated with the observed upper-level divergence over Texas (Fig. 3.37.c), and continued the advection of tropical moisture into the region. Closed circulation, associated with the 500 mb low, was also now evident over Baja California.

Figures 3.60.a-c and 3.61 a-d show the streamline patterns at 0000 GMT 20 October. Two main zones of low-level (sfc, 850 mb) convergence were now evident at this time. The first being located from northern sections of the Permian Basin (MAF), extending east-northeastward to north-central Texas. This convergence was along the cold frontal boundary presently located over this region. This zone was reflected by the maximum in the surface moisture convergence field over central Texas at this time (Fig. 3.46). This maximum strengthened during the past 12 hours. The second area of convergence could be seen over central Mexico, south of CUU. This convergence was associated with the circulation of the remnants of Hurricane Tico. Recall that Tico made landfall upon the west coast of Mexico 12 hours earlier. The flow above 850 mb began to change during the past 12 hours. At 700 mb, the flow was now south-southwesterly, becoming more influenced by 158

STREAMLINES aez 2fl OCT 1983 STREAMLINES eaz 2e OCT i983

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STREAMLINES aez 2a OCT I983

7ea ne Figure 3.60. Streamline objective analysis for 0000 GMT 20 October at a) surface, b) 85(J mb, and c) 700 mb. 159

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the west coast long wave trough. This prohibited Gulf moisture from further entering west Texas. Diffluence in the streamline pattern was now taking place over western Texas and eastern New Mexico. This was reflected by the 700 mb divergence located over a large portion of southwest Texas at 0000 GMT (Fig. 3.37.c). Closed circulations were now evident over southern Arizona from SOOmb to 300 mb, with the circulation centers tilted to the west-northwest with height. Broad southwesterly flow continued over Texas and Oklahoma, resulting in the continuation of Pacific, tropical moisture advectlon. The anticyclonic curvature in the upper-level streamline pattern persisted over this area, with slight diffluence also evident at 300 mb. This again was reflected by horizontal divergence present at these levels (Fig. 3.38a-d). Figures 3.62a-c and 3.63a-d show the streamline patterns at 1200 GMT 20 October. The main feature at the lower-levels (sfc, 850 mb) was the cyclonic circulation center over central Texas. This circulation was associated with the remnants of Hurricane Tico, now an extratropical cyclone. Surface convergence, extending south and northeast of the circulation center, was also evident. This convergence was associated with the frontal boundaries presently located over this region. The circulation center, and frontal boundaries were also reflected by the maximum in the surface moisture convergence field (Fig. 3.47). A broad cyclonic circulation was evident at 700 mb. the circulation extended from central Arizona to western Texas, with the center located near ELP. Dif luence at this level was present over the eastern Texas Panhandle and Oklahoma as a result of the cyclonic circulation. Slight 700 'z\

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SURFACE 8S0 MB

12Z 20 OCT 1983 STREAMLINES

700 na Figure 3.62. Streamline objective analysis for 1200 GMT 20 October at a) surface, b) 850 mb, and c) 700 mb. 162

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3ee MB 2ea MB

Figure 3.63. Streamline objective analysis for 1200 GMT 20 October at a) 50(3 mb, b) 40(5 mb, c) 300 mb and 200 mb. 163 mb divergence over this region resulted from the difluent pattern (Fig. 3.39c). The 500 mb closed circulation was now positioned on Arizona-New Mexico, with the associated long-wave trough extending southward into central Mexico. The circulation center traveled northeastward over the past 12 hours. Closed circulations were no longer evident at 400 and 300 mb; however, distinct negatively-tilted troughs were now located at both levels over Arizona and New Mexico. These upper-level streamline patterns resulted in the continuation of southwesterly flow, hence tropical moisture advection, over Texas and Oklahoma. The anticyclonic curvature to the streamlines persisted, while shifting to the east-northeast. The combination of the negative-tilt troughs and anticyclonic curvature resulted in diffluence over northern Texas and Oklahoma at the 400, 300, and 200 mb levels. This was again reflected by the upper-level horizontal divergence present over this region at 1200 GMT (Fig. 3.40b-d). These streamline and associated convergence/divergence patterns would continue to evolve and travel east-northward over the next 24-48 hours, eventually exiting the rain-stricken areas of Texas and Oklahoma. CHAPTER IV CONCLUSIONS

During the time period 17-21 October 1983, a heavy rain event took place from the South Plains of Texas to central Oklahoma, as the remnants of Northeast Pacific Hurricane Tico crossed over this area. Rainfall amounts of up to 11 inches was recorded over south central Oklahoma, while precipitation in excess of 7 inches was observed over the South Plains. Several surface and upper air synoptic features initiated the heavy rain event 24-36 hours prior to the arrival of Tico's remnants. At the surface, throughout the period, a combination of warm, cold, and quasi-stationary frontal boundaries provided continuous focusing mechanisms, along which the heavy precipitation was aligned. The remnants of Tico interacted with a cold frontal boundary over West Texas, resulting in the transformation of the tropical cyclone into an extratropical cyclone. Low-level southeasterly flow from the Gulf of Mexico provided an abundant supply of moisture prior to and during the transformation of the cyclone. Strong overrunning conditions along the frontal boundaries resulted in a majority of the heavy rain. Supporting the lower-level features was a broad, upper-level long wave trough positioned over the western coasts of the U.S. and Mexico. This long wave trough was evident at 700 mb, 500 mb, and 200 mb. This resulted in strong southwesterly flow over Texas and Oklahoma, along with the advection of Pacific tropical moisture at these levels, throughout the heavy

164 165 rain event. A series of short wave troughs, embedded within the wave at 700 mb and 500 mb, accompanied with a strong 200 mb polar jet, also advected through Texas and Oklahoma during the heavy rain episode. The atmosphere over Texas and Oklahoma became increasingly moist and unstable prior to and during the heavy rain event. These were reflected by increases in the surface-500 mb average relative humidity and precipitable water, along with values representative of unstable conditions in both the Lifted Index (LI) and K-lndex (Kl). Objective analyses of mixing ratio from the surface up to 500 mb also indicated an abundant supply of low-level moisture, along with larger concentrations of mid- and upper-level moisture, at the time of the largest rainfall amounts over the South Plains. Strong surface moisture convergence was evident throughout West Texas and Oklahoma, with maximum concentrations of low-level moisture located in the regions of heavy precipitation. This maximum area of surface moisture convergence propagated eastward during the period, reflecting the shift in the low-level moisture as the remnants of Tico traveled through the region. Objective analyses of horizontal divergence from the surface up to 200 mb revealed a general pattern of lower-level convergence and upper-level divergence over Texas and Oklahoma. This pattern was conducive to 500 mb upward vertical motion, which was evident over the heavy rain region throughout the time period. Upward vertical velocities intensified with time over northern Texas and Oklahoma, coinciding with the period of heaviest rainfall over this area. 166

Objective analyses of 500 mb absolute vorticity Indicated slight positive vorticity advection (PVA) over New Mexico, Texas, and eventually Oklahoma through the period. This vorticity gave additional evidence of the presence of short waves located over this area. Stronger positive vorticity advectlon occurred after the abundance of low-level moisture exited the region. Finally, a streamline objective analysis revealed strong, low-level southeasterly flow off the Gulf of Mexico throughout a majority of the period, with north-northeasterly flow occurring north of the frontal boundaries. This resulted in a low-level convergence zone over West Texas and Oklahoma. The streamline pattern veered with height resulting in southwesterly flow at the upper-levels. This flow pattern resulted in the advection of Pacific, tropical moisture over the heavy rainfall regions of Texas and Oklahoma. Lubbock received approximately one-third of its 1983 rainfall during this event. Lubbock received 20.23 inches of precipitation for the entire year, with 10.80 inches coming in the month of October. Lubbock recorded over 7.3 inches during the heavy rain event. LIST OF REFERENCES

Allard, R.A 1984: A climatology of the characteristics oftropical cyclones in the Northeast Pacific during the period 1966-1980 MS thesis Atmospheric Science Group. Texas Tech University. Lubbock, TX.,' 106 pp.

Bosart, L.F., and F.H. Carr, 1977: A case study of excessive rainfall centered around Wellsville, New York, 20-21 June 1972. Mon. Wea. Rev.. 106,348-362. Byrd, G.P., and E.W. McCaul, Jr. 1985: Analysis of heavy rainfall and severe weather associated with the remnants of Hurricane Tico. Preprints, 14th Conference on Severe Local Storms, J 33-35. Grice, G.K., and Maddox, R.A., 1982: Synoptic aspects of heavy rain events in South Texas associated with the westerlies. NOAA Tech. Memo.. NWS SR-106, 21 pp. Johnson, G.A., and E.B. Mortimer, 1983: Effects of Eastern Pacific cyclone moisture on a heavy rain and flood event in Texas and Oklahoma. National Weather Digest. 9, No. 4. 16-29. U.S. Department of Commerce, October 1983: Hourly Precipitation Data. Texas and Oklahoma. 33. No. 10. 46 pp. U.S. Department of Commerce, October 1983: Storm Data. 25. No. 10. 25 DP.

167 APPENDICES

A. RAWINSONDE STATIONS FOR U. S. AND MEXICO B. DEFINITION OF K-INDEX AND LIFTED INDEX

168

Hi,* 169 APPENDIX A

APPENDIX A: RAWINSONDE STATIONS FOR U.S. AND MEXICO

ABQ Albuquerque, NM JAN Jackson, MS

AHN Anthens, GA LBF North Platte, NE

AMA Amarillo, TX LCH Lake Charles, LA

AQQ Apalachicola, FL LIT Little Rock, AR

BNA Nashville, TN LND Lander, WY

BRO Brownsville, TX MAF Midland, TX

BVE Boothville, LA MTY Monterrey, MX

CUU Chihuahua, MX MZT Mazatlan, MX

DAY Dayton, OH OKC Oklahoma City, OK

DDC Dodge City, KS OMA Omaha, NE

DEN Denver, CO PIA Peoria, IL

DRT Del Rio, TX RAP Rapid City, 5D

ELP El Paso, TX SAN San Diego, CA

ELY Ely, NV SEP Stephensville, TX

EMP Empalme, MX 5LC Salt Lake City, UT

FNT Flint, Ml SLO Salem, IL

GGG Longview, TX STC St. Cloud, MN

GJT Grand Junction, CO TOP Topeka, KS

GRB Green Bay, Wl TUS Tucson, AZ

GUE Guadalupe Island, MX UMN Monett, MO

HT5 Huntington, WV VCT Victoria, TX

INW Winslow, AZ 170 APPENDIX B:

DEFINITION OF K-INDEX (K-l) AND LIFTED INDEX (LI)

Kl = T850 " Td850 " "^700 * TCI700 " "^500 where: JQ^Q = temperature at 850 mb CO Tdsso ^ viewpoint at 850 mb CO T700 = temperature at 700 mb CO TdyQQ = dewpoint temperature at 700 mb CO T500 = temperature at 500 mb CO K-lndex <15 15-20 21-25 26-30 31-35 36-40 >40 (%) Probability of 0 <20 20-40 40-60 60-80 80-90 ^100% thunderstorms

L'"'^®500'''"P500 where: Te^QQ = environmental temperature at 500 mb CO TP500 = parcel temperature at 500 mb CO after lifting moist adiabatically from the lifting condensation level (LCD.

LI Conditions > + 3 stable, no convective activity *1 to+3 marginally unstable, showers -3toH unstable, thunderstorms -6 to -3 very unstable, severe thunderstorms <-6 tornadic potential PERMISSION TO COPY

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