Lightning During Two Central US Winter Precipitation Events

DECEMBER 1996 NOTES AND CORRESPONDENCE 599

NOTES AND CORRESPONDENCE

Lightning during Two Central U.S. Winter Precipitation Events

RONALD L. HOLLE AND ANDREW I. WATSON National Severe Storms Laboratory/NOAA, Norman, Oklahoma 17 October 1994 and 15 June 1996

ABSTRACT Network-detected cloud-to-ground lightning coincident with mainly frozen precipitation (freezing rain, sleet, snow) was studied over the central United States during two outbreaks of arctic air in January 1994. During the ®rst event, the ratio of positive to total ¯ashes was 59%, ¯ashes were few and disorganized in area, and no surface observer reported thunder. For the other event the ratio was 52% during the ®rst few hours in subfreezing surface air, then decreased when ¯ashes formed in the nearby region above freezing. Also, ¯ashes in this case were linearly aligned and coincided with conditional symmetric instability; thunder was heard infrequently by surface observers. On radar, re¯ectivity cores grew from weak to moderate intensity within a few hours of the lightning during both cases. Echo area increased greatly before ¯ashes in one case, while the area increase coincided with ¯ashes in the other. Some base-scan re¯ectivities were strong in both thunderstorm regions due to the radar beam intersecting the melting level. Regions with lightning often could be identi®ed better by high echo tops than re¯ectivity. Analyses on the scale of one or two states diagnosed the strength of low-level warming that contributed to formation of thunderstorms and signi®cant frozen precipitation. Quasigeostrophic analyses showed that 850-mb temperature advection and 850±500-mb differential vorticity advection were similar in magnitude in the lightning area during both events. Once convection formed, lightning and echo-top information identi®ed downstream regions with a potential for subsequent frozen precipitation.

1. Introduction the isolated event coincided with, or were within 150 km of subfreezing air and frozen precipitation at the During January 1994, a series of arctic fronts swept ground. About 20% of the ¯ashes in the more organized from northwest to southeast across the eastern two-thirds event were in such regions. of the United States. Low-temperature records for the day, Studies of simultaneous frozen precipitation at the the month, and for all time were set across the Midwest surface and CG lightning detected by networks were and Great Lakes states, especially on 4±6 and 16±19 reviewed by Holle and LoÂpez (1993). Frequent ¯ashes January, according to the National Oceanic and Atmo- were associated with the large and intense March 1993 spheric Administration (NOAA) publication Storm Data. snowstorm on the U.S. east coast (Orville 1993). Other Storm Data is a monthly publication prepared by the Na- studies usually have been of the ¯ow of cold air over tional Climatic Data Center (NCDC) in Asheville, North large unfrozen water bodies. A few ¯ashes were found Carolina. Ahead of each arctic outbreak developed south- downwind of the Great Lakes in convective snowbands erly component winds above the surface, frozen precipi- by Moore and Orville (1990). Large numbers of tation at the ground (freezing rain or drizzle, sleet, or ¯ashes occurred as cold air ¯owed over the Gulf Stream snow), and thunderstorms. (Biswas and Hobbs 1990; Orville 1990, 1993) and the Two lightning events during January 1994 over Mis- Sea of Japan (Goto et al. 1992). souri and Arkansas coincided with subfreezing tem- Winter thunderstorms have also been examined peratures and frozen precipitation at the ground. One without lightning network data. Curran and Pearson isolated event had 27 cloud-to-ground (CG) ¯ashes (1971) calculated a mean sounding near 76 thun- over Missouri, and the other more organized event had dersnow reports and found a subfreezing layer near 2417 ¯ashes from Oklahoma into Arkansas. Flashes in the ground, topped by an inversion whose upper-half was warmer than freezing, then a deep layer with high relative humidities. Beckman (1987, 1989) and Corresponding author address: Ronald L. Holle, National Severe Elkins (1989) suggested more study of thundersnow Storms Laboratory, NOAA, 1313 Halley Circle, Norman, OK 73069. in the context of using satellite imagery for heavy E-mail: [email protected] snow forecasting. Colman (1990a,b) examined the

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2. Data The CG lightning data were collected by the Na- tional Lightning Detection Network (NLDN) described by Cummins et al. (1995) and Orville (1994). The NLDN during January 1994 was composed of direction ®nder antennas (Krider et al. 1976, 1980; Holle and LoÂpez 1993). Location accuracy for ¯ashes within the NLDN at this time was estimated as 5±10 km. Sys-

FIG. 1. Missouri counties and 230-km (124 n mi) ranges from Kansas City (EAX, shaded) and St. Louis (LSX) radars. Negative cloud-to-ground lightning ¯ashes from 1003 to 1340 UTC 10 January 1994 are shown by a square; positive ¯ashes by a /. UMN is Monett, Missouri upper-air station. Position of 0ЊC surface isotherm shown by dashed line for center of lightning period. dynamic and thermodynamic environments of elevated thunderstorms in the eastern United States on the cold side of fronts. Stewart and King (1990) con- sidered the region separating frozen from liquid precipitation in southern Ontario; thunder was heard by observers in one of two cases. Galway and Pearson (1981) focused on winter tornadoes in the central United States that usually had widespread blizzards, heavy snow, and/or extensive glazing on the cold side of the weather systems. Grant (1995) studied severe spring thunderstorms in the cold sector north of fronts without coincident frozen precipitation at the ground. Marwitz and Toth (1993) determined the synoptic-scale kinematic ®elds and radar structure of a warm-frontal snowband over Oklahoma ahead of a surface cold front; thunder was observed but was not the focus of the study. Beckman (1989) and Colman (1990a) recommended the use of lightning data to study winter thunder observations. The National Weather Service (NWS) Storm Pre- diction Center (SPC) is being established in Norman, Oklahoma (McPherson 1994), to provide guidance, coordination, and short-term forecasting for winter weather across the United States. The extent to which lightning data at SPC and other NWS of®ces can iden- FIG. 2. Surface maps for middle Mississippi Valley at 1300 (upper tify locations and times of signi®cant frozen precipi- panel) and 1500 UTC (lower panel) 10 January 1994. Station models tation is a motivation for the present study. This study and all symbols are conventional; temperature and dewpoints in ЊF; full wind barb Å 5ms01 (10 kt); half barb Å 2.5 m s 01 (5 kt). will compare lightning with radar and surface data, and Pressure contours (solid) are in mb. Shading and ZR indicate regions synoptic- and subsynoptic-scale conditions at times and with mainly freezing rain; S- for mainly snow. Warm front in south- places of ¯ashes. west part of map.

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Unauthenticated | Downloaded 10/02/21 11:39 PM UTC DECEMBER 1996 NOTES AND CORRESPONDENCE 601 tem detection ef®ciency, the ratio of ¯ashes detected WSR-88D radar re¯ectivity and echo-top heights at by the network to the number of CG ¯ashes that actu- several sites were obtained in near-real time as products ally occurred, was estimated as 65%±80% for the from WSI Corporation (a private vendor). NWS fore- NLDN. Accuracy and detection ef®ciency are based on casts and discussions were obtained from the Experi- widely available sources such as Orville (1993, 1994), mental Forecast Facility collocated at the Norman since no independent ground truth was collected at the NWS Forecast Of®ce (NWSFO) described by Janish times and locations of the storms. et al. (1995).

FIG. 3. Radar re¯ectivity from WSR-88D radar at Kansas City (EAX) at 0900 (top) and 1200 UTC (bottom) 10 January. Circle is 230-km (124 n mi) range from radar.

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FIG. 4. Radar re¯ectivity from WSR-88D radars at Kansas City (EAX, top) and St. Louis (LSX, bottom) at 1500 UTC 10 January. Circle is 230-km (124 n mi) range from radar.

3. 10 January 1994 case ¯ashes lowering positive charge to ground and 11 negative ¯ashes from 1003 to 1340 UTC where sur- a. Lightning face temperatures are slightly above freezing. There The NLDN detected 27 CG ¯ashes in western is no recognizable con®guration to the ¯ashes, Missouri on 10 January (Fig. 1). There are 16 which differs from thunderstorms in Colman

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(1990a,b) that originated in warm air south of warm was reported within both regions at 1500 UTC. Several fronts and moved north over subfreezing surface stations in the lightning region reported sleet (ice pel- temperatures. lets) at and just above the freezing point between 1000 The ratio of 59% positive ¯ashes is much higher and 1200 UTC, and freezing rain was observed in Mis- than the 4% detected over the entire United States souri starting at 1300. Surface temperatures were above during 3 years (Orville 1994); the ratio is usually freezing where lightning occurred, but freezing tem- lowest in summer. High ratios of positive ¯ashes peratures at the ground were less than 50 km to the were noted late in the life cycle of large mesoscale northeast. Large temperature±dewpoint spreads across convective systems (Holle et al. 1994), during some much of Fig. 2 indicate that evaporative cooling from severe storms (Knapp 1994), and during cold con- falling precipitation may inhibit warming to above ditions aloft (Goto et al. 1992). Prior results near freezing during the day. large water bodies showed that a majority of ¯ashes After lightning ended, mainly freezing rain moved were positive (Moore and Orville 1990) and that a northeast across Missouri. St. Louis's Lambert Field 13% ratio existed over a broad region during the had poor braking action, and several runways were East Coast blizzard of 1993 (Orville 1993). A more closed for short periods for salt application. There were complete review of positive ratios is in Holle and morning traf®c disruptions in St. Louis until tempera- LoÂpez (1993). tures rose above freezing late in the morning; driving and walking conditions in the northern half of Missouri b. Winter precipitation were described in the 0700 CST/1300 UTC NWS A moderate pressure gradient is producing south statement from Columbia, Missouri, as ``extremely winds across the surface maps of Fig. 2. A north±south treacherous.'' Frozen precipitation fell across central cold front is just off the map to the west, and a weak and northern Illinois later in the day, but there was no warm front is located in the southwest portion of the lightning. map. No surface observing station in the region reported thunder or lightning during this event, but it is c. Radar not especially unusual to have few observer reports of lightning or thunder in storms with low ¯ash rates Two WSR-88D radars, one near Kansas City (Pleas- (Reap and Orville 1990; Changnon 1993). ant Hill, Missouri; EAX) and one near St. Louis (Wel- Shaded areas in Fig. 2 show where freezing rain or don Spring, Missouri; LSX) observed the storm. Figure light snow was reported by observers; sleet (ice pellets) 3 from Kansas City shows small, short lines of radar

FIG. 5. Radar echo tops from Kansas City WSR-88D (EAX) radar at 1500 UTC 10 January. Circle is 230-km (124 n mi) range from radar.

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FIG. 6. Temperature and dewpoint (ЊC) soundings at Monett, Missouri, at 0000 (left) and 1200 UTC (right) 10 January 1994, plotted on a skew T±logp diagram. Pressure (vertical axis) is labeled in mb. Solid lines are temperature and dewpoint temperature (ЊC). Winds are shown by ¯ag Å 25 m s 01 (Ç50 kt); full barb Å 5ms01 (10 kt); half barb Å 2.5 m s 01 (5 kt). re¯ectivity in western Missouri at 0900 and 1200 UTC the radar beam intersecting the melting level at the 0.5Њ (lightning occurred from 1003 to 1340 UTC). Re¯ec- elevation angle from 50 to 75 n mi (90±140 km) from tivities are mostly 20±30 dBZ but are above 35 dBZ in the radar. The Monett sounding (Fig. 6, section 3d, a few areas. The area of returns above 30 dBZ grew, right) is warmer than freezing from the surface to reached a maximum, and dissipated during nearly the higher than 800 mb. Farther north in Missouri, the same time as the lightning. highest levels with temperatures above freezing are be- At 1500 UTC, Fig. 4 shows re¯ectivity from both tween 0.8 and 1.2 km according to the radar re¯ectivity radars, and Fig. 5 shows echo tops in western Missouri maximum (Fig. 4). Stewart and King (1990) also up to 5.8 km (18 000 ft). Re¯ectivity echoes are more noted re¯ectivity bright bands below the 0ЊC level in continuous in eastern Missouri and western Illinois an Ontario winter storm. There are no large echoes with than a few hours earlier. The convective elements em- strong re¯ectivities and isolated cores (Figs. 3±5) that bedded in the stratiform precipitation are better iden- are typically expected with thunderstorms. ti®ed by highest echo tops east of Kansas City. Echo tops in central Missouri reached 6.1 km (20 000 ft) on d. Warm-layer and instability formation the St. Louis radar (not shown). These heights are in the lowest range of echo tops coincident with network- Soundings from Monett are shown in Fig. 6. At 0000 detected ¯ashes during a summer squall line in UTC 10 January, temperatures are warmer than freez- Oklahoma (Watson et al. 1995). Over eastern Mis- ing below 850 mb (Fig. 6, left) but dry up to 550 mb. souri, the radar at St. Louis detects re¯ectivities ex- Twelve hours later, a layer warmer than freezing exists ceeding 35 dBZ over larger areas than the Kansas City up to 800 mb that is nearly saturated to 650 mb (Fig. radar. Both indicate somewhat concentric features that 6, right). Temperatures above 600 mb are nearly un- have different orientations in eastern Missouri. Higher changed from 12 h earlier. A saturated parcel was lifted re¯ectivities in the mainly north±south band in central from 835 mb at the top of the warm layer on the 1200 and northern Missouri (Fig. 4, bottom) are ascribed to UTC Monett sounding. The maximum temperature dif-

TABLE 1. Temperatures at 850, 700, and 500 mb, and temperature differences between levels on 9, 10, and 11 January 1994. Temperatures apply to center of lightning region in western Missouri (Fig. 1). Bold entries are during lightning from 1003±1340 UTC 10 January 1994.

Time (UTC/date) 850-mb T 700-mb T 500-mb T 850±500 DT 700±500 DT

1200/09 Jan. 04ЊC 09ЊC 022ЊC18ЊC13ЊC 0000/10 Jan. /1ЊC 07ЊC 022ЊC23ЊC15ЊC 1200/10 Jan. /3ЊC 07ЊC 023ЊC26ЊC16ЊC 0000/11 Jan. 01ЊC 05ЊC 024ЊC23ЊC19ЊC

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Unauthenticated | Downloaded 10/02/21 11:39 PM UTC DECEMBER 1996 NOTES AND CORRESPONDENCE 605 ference from the observed is 3ЊC at 700 mb, which is a lifted index of 03 by standard sign convention. Southwest winds reach 28 m s 01 in this layer at 1200 UTC. North of Monett, in the lightning region over western Missouri, the steepest 850±500-mb lapse rate occurred at the time of lightning (Table 1). This lapse rate was due mainly to warming near 850 mb, while there was only slight cooling at 500 mb. Pro®ler winds at 1500 m and 850-mb data from upper-air stations at 1200 UTC 10 January are shown by Fig. 7 in the middle of the lightning period. Southwest winds exceed 15 m s 01 (30 kt) over a large region and 26 m s 01 (50 kt) in the area just upwind from the lightning. The St. Louis NWS discussion at 1000 CST/1600 UTC 10 January stated that, ``0000 UTC model guidance did a poor job handling the overnight warming of the low levels due to a low-level jet exceeding 50 kt (26m s01) from the Southern Plains into southwestern Missouri.'' Quasigeostrophic analyses at 1200 UTC 10 January are shown in Fig. 8. The top panel shows contributions to vertical motion due to differential vorticity advection 01 0 f0s Ì/Ìp[0£g·Åp (zg / f )] in the 850±500-mb layer, where f and s are the Coriolis and static stability parameters and £g and zg are geostrophic velocity and relative vorticity. Static stability was calculated at each grid point. The lower panel shows contributions to

FIG. 8. Upper-air data diagnosed at 1200 UTC 10 January over the south central United States scaled by 10 015 kPa s 01 m02 . Positive values contribute to upward vertical motion. (Top) Differential vorticity advection in 850±500-mb layer. (Bottom) Temperature advection at 850 mb.

vertical motion due to the temperature advection 01 012 0RspÇpgp(0£·ÅT)pattern at 850 mb, where R is the universal gas constant. Both terms are scaled by 10 015 kPa s 01 m02 and have contributions to upward motion in a north±south orientation in Missouri where lightning occurred. Twelve hours earlier (not shown), differential vorticity advection provided no contribution to vertical motion, while temperature advection contributed to downward motion over eastern Oklahoma and Kansas and upward motion to the west.

FIG. 7. Winds at 1500 m from pro®lers (square) and 850-mb wind, e. Forecast issues temperature (upper number), and dewpoint (lower number) at upper-air stations (circle) at 1200 UTC 10 January. Winds ú 25 m s 01 (50 kt) are hatched diagonally; winds ú 15 m s 01 (30 kt) are shaded. The NWS National Severe Storms Forecast Center Temperatures at 850 mb are indicated by dashed isotherms. Thin in Kansas City on the morning of 10 January included north±south dashed line is wind shift line. the possibility of thunderstorms in the region and

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FIG. 9. Lightning in the south-central United States by 4-h intervals from 1800 UTC 16 January to 1000 UTC 17 January 1994. Negative ¯ashes are shown by small /; positive ¯ashes by large /. MLC is McAlester, Oklahoma; FSM is Fort Smith, Arkansas, and LZK is Little Rock, Arkansas. Position of 0ЊC surface isotherm shown by dashed line for the center of each time period.

noted that a few strikes were in western Missouri. At today . . . once we see where it gets organized may 0620 CST/1220 UTC 10 January, the Kansas City have to go with advisory, however temperatures will NWS forecast mentioned the likelihood over the next remain generally above freezing today.'' The main hour of ``a few thunderstorms producing brief peri- concern was with snow more than freezing rain. At ods of heavy sleet.'' Local NWS stations had access 0955 CST/1555 UTC 10 January the St. Louis dis- to gridded ¯ash information, but not individual cussion mentioned the threat of frozen precipitation ¯ashes as at the center. At 0345 CST/0945 UTC 10 due, in part, to evaporative cooling but expected it to January, the St. Louis forecast discussion anticipated end quickly through the day. Since no lightning or that there would be ``a mixed bag of precipitation thunder was reported at any hourly observing site,

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4. 16±17 January 1994 case a. Lightning The NLDN located 2417 CG ¯ashes during 16 h in the south central United States starting at 1800 UTC 16 January (Fig. 9). During the ®rst 4 h, 21 of 25 ¯ashes were in areas colder than freezing at the surface (Fig. 9a); the scattering of ¯ashes resembled the 10 January case. During the next 4 h (Fig. 9b), lightning began in areas warmer than freezing, ended in colder areas to the northeast, and tended toward a linear or banded structure from southwest to northeast. All ¯ashes after 0200 (Figs. 9c,d) were in regions of above-freezing temperatures. Lightning was in a similar con®guration to thunderstorms in Colman (1990a,b) that originated south of the warm front and continued above the frontal surface. FIG. 10. Surface map for the southern plains at 2300 UTC 16 During the ®rst 4 h, 52% of the ¯ashes were positive January 1994. Cold front in northwest part of map, and warm front when lightning was mainly in regions with subfreezing to south. Other symbols as in Fig. 2. surface temperatures. A value of 59% occurred on 10 January (Fig. 1), where surface temperatures were a few degrees above freezing. Whether a high ratio oc- sas had calm winds at 2300 UTC (Fig. 10) and for curred because of cold surface conditions or it was the several hours around this time when adjacent stations early stage of a storm is not clear from two cases. For reported freezing rain or rain; the wind sensors appar- the 16-h period, 29% of the ¯ashes lowered positive ently were frozen due to accumulated ice. No present charge to ground. There was a tendency for positive weather was observed or transmitted by these AWOS ¯ashes to be more frequent on the northeast ends of stations. individual lines from 2200 UTC 16 January to 0600 17 After 0100 UTC 17 January, freezing rain was in a January. A higher ratio of positive to total ¯ashes to- band from northern Arkansas into southern Indiana. ward the end of a storm is often found in less severe Snow was reported to the north, while rain was reported convection, such as mesoscale convective systems to the south where surface temperatures were above (Holle et al. 1994) and winter thunderstorms along the freezing (0ЊC isotherm is in Fig. 9). The Oklahoma Gulf Coast (Orville et al. 1988), but severe storms can City NWSFO on the afternoon of 16 January stated be dominated by positive ¯ashes through a portion or that, ``roadways throughout northeast and east-central all of their lifetime (Knapp 1994; MacGorman and Oklahoma remain icy and hazardous. Travel is dis- Burgess 1994). couraged.'' NOAA's Storm Data reported widespread accumulations of ice and snow up to 7.6 cm (3 in.) in b. Winter precipitation eastern Oklahoma, 12.7 cm (5 in.) in northern Arkan- sas, and 20 cm (8 in.) in northeast Arkansas. Storm A weak pressure gradient produced light southerly Data reported a large number of trees and power lines winds across the surface map of Fig. 10 at 2300 UTC. knocked down by the weight of ice and snow, and A cold front is on the edge of the map to the northwest, 15 000 electric customers without power at the height and a warm front is across the southern border of Ar- of the storm. Traveling was described by the Little kansas. Fort Smith, Arkansas (FSM), reported thunder, Rock NWSFO as ``almost impossible in many areas'' freezing rain, and fog at 2200 and 2300 16 January in the northern half of Arkansas. (Fig. 10) and at 0000 17 January. Freezing rain was light at 2200 and moderate at 2300 and 0000. Mc- c. Radar Alester, in eastern Oklahoma, reported a surface temperature of 1ЊC(34ЊF) with thunder and rain at 0300 The Little Rock radar at 0005 UTC 17 January (Fig. UTC 17 January. Snow was falling over Missouri, and 11, top) has scattered echoes in southern Arkansas and one observation of sleet (ice pellets) was in southern more continuous echoes to the north; several short lines Illinois. Warmer temperatures, fog, and rain were south of higher re¯ectivity are evident. Frequent lightning be- of frozen precipitation. Three Automatic Weather Ob- gan in western Arkansas during the previous 2 h (Fig. serving System (AWOS) stations in northeast Arkan- 9). Re¯ectivities exceed 35 dBZ in several regions to

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FIG. 11. Radar re¯ectivity from the Little Rock WSR-88D (LZK) at 0005 (top) and 0500 UTC (bottom) 17 January. Circle is 230-km (124 n mi) range from radar.

the north, and one small area exceeds 50 dBZ within a case and are near the typical echo tops coincident with large area of strong returns. However, convective ele- ¯ashes during a summer squall line in Oklahoma (Wat- ments embedded in the stratiform precipitation are bet- son et al. 1995). ter located by echo tops (Fig. 12, top), as in the other Five hours later at 0500 UTC (Fig. 11, bottom), ra- event (Fig. 5). Cell tops exceed 11 km (30 000 ft) in dar shows a large area of re¯ectivity exceeding 35 dBZ; western Arkansas where lightning was occurring (Fig. some areas exceed 50 dBZ. Freezing rain, snow, and 9b). These heights are much greater than in the ®rst rain are reported at this location in Fig. 10. A few echo

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FIG. 12. Radar echo tops from the Little Rock WSR-88D (LZK) at 0005 (top) and 0500 UTC (bottom) 17 January. Circle is 230-km (124 n mi) range from radar.

tops continue over 11 km (30 000 ft) in western Ar- ascribed to the radar beam intersecting the melting level kansas (Fig. 12, bottom) in the lightning area (Fig. 9c). at a 0.5Њ elevation angle at 40±70 n mi (75±130 km) The strong echo that developed 75 km north of Little from the radar. Soundings at Monett (north of the high Rock at 0500 UTC is east of the cold front and north re¯ectivities) indicate a layer from 0.7 to 1.1 km where of the warm front. High re¯ectivities in the east±west temperatures are a little warmer than freezing accord- region across northern Arkansas (Fig. 11, bottom) are ing to the location of the radar re¯ectivity maximum

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FIG. 13. Little Rock soundings at 1200 UTC 16 January (left), and 0000 (center) and 1200 UTC (right) 17 January, as in Fig. 6.

(Fig. 11). The extensive area of strong re¯ectivities ¯owing northward over the shallow cold air mass over was due to melting precipitation and had no lightning the central plains.'' or exceptionally high echo tops (Fig. 12, bottom). In There was strong southerly component ¯ow of sat- the previous case, lightning occurred at about the same urated air with above-freezing temperatures through a time as the increase in areal radar coverage and the start deep layer over cold surface air that produced freezing of moderate re¯ectivity cores. For this event, the area rain; these features are similar for both January 1994 of echoes was extensive when stronger re¯ectivity events. The maximum temperature difference at Little cores began at the time of ®rst ¯ash at 1738 UTC. Rock was 1.5ЊC (lifted index of 01.5 by standard sign convention) at 685 mb when a parcel was lifted from d. Warm-layer and instability formation the top of the warm layer at 0000 UTC. The 850±500- mb lapse rate was 22ЊC at Fort Smith when thunder and A series of three soundings at Little Rock (Fig. 13) freezing rain was reported and 17ЊC from 700 to 500 shows that at 1200 UTC 16 January (left) a shallow mb (Table 2). The 850±500-mb lapse rate was slightly above-freezing layer is between 800 and 875 mb. greater 12 h earlier at Fort Smith (Table 2). Steepest Strong veering of the wind, implying warm advection, lapse rates are due to warming in the 850±700-mb is evident at low levels. Twelve hours later (center), a layer and not to cooling at 500 mb. Trends and lapse deeper layer from 725 to 950 mb exceeds 0ЊC, while rates of temperature through the period are similar to surface temperatures continue below freezing. West- those in the lightning area during the 10 January case, southwest ¯ow at 25±36 m s 01 in this layer strength- but temperature differences are not as large on 16±17 ened at Little Rock during the day. All levels at Little January. Rock are nearly saturated, while it was dry above 700 A north±south cross section from near Kansas City mb 12 h earlier. Large-scale ascent may have assisted into the Gulf of Mexico at 0000 UTC (Fig. 14) during the moistening. After another 12 h (right), the cold the time of lightning shows conditional symmetric in- front passed Little Rock and winds below 600 mb were stability (CSI) to the north below 800 mb (shaded). from north to northwest, all temperatures were below As required for CSI (Sanders and Bosart 1985; Snook freezing, and drier conditions existed above 800 mb. 1992; Moore and Lambert 1993), conditions are near The forecast discussion from the Oklahoma City saturation, there is little directional shear in the vertical, NWSFO at 2110 CST 15 January/0310 UTC 16 Jan- and ue lines are somewhat more vertical than momen- uary pointed out that, ``rich low-level moisture was tum lines. The linear lightning paths at this time and

TABLE 2. Temperatures at 850, 700, and 500 mb, and temperature differences between levels on 16 and 17 January 1994. Temperatures apply to center of lightning region around Fort Smith, Arkansas (Fig. 9). Bold entries are during thunderstorm with freezing rain observed at Fort Smith.

Time (UTC/date) 850-mb T 700-mb T 500-mb T 850±500 DT 700±500 DT

0000/16 Jan. 01ЊC 08ЊC 020ЊC19ЊC12ЊC 1200/16 Jan. /4ЊC 06ЊC 019ЊC23ЊC13ЊC 0000/17 Jan. /3ЊC 02ЊC 019ЊC22ЊC17ЊC 1200/17 Jan. 03ЊC 07ЊC 024ЊC21ЊC17ЊC

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FIG. 14. South±north cross section from the Gulf of Mexico on the south at 25ЊN, 95ЊW (right) to Kansas City on the north at 40ЊN, 95ЊW at 0000 UTC 17 January. Shaded regions indicate where criteria are met for CSI. Region below thick-dashed line indicates area with potential for convective instability. Relative humidity is dotted (tens of percent), angular mo- 01 mentum is solid (m s ), and ue is dashed (K).Pressure (vertical axis) is labeled in mb. Location of cross section is along horizontal axis in latitude (top number in degrees and hundredths) and longitude (bottom numbers).

location (Fig. 9) track individual thunderstorms or a with snow or freezing rain from Texas into northern succession of convective updrafts that formed in the Arkansas and western Tennessee in its 0100 CST/ conditionally unstable environment and moved north- 0700 UTC 16 January convective outlook. The Tulsa east (Fig. 13, middle). Intense winter precipitation oc- NWSFO area forecast discussion at 0425 CST/1025 curring in bands and/or lines was correlated with CSI UTC 16 January remarked that, ``gridded data show by Sanders and Bosart (1985), Howard and Tollerud signi®cant instability developing above the surface (1988), Snook (1992), and Moore and Lambert with Total Totals of 48 to 50 in southeast Oklahoma (1993). Browning and Foster (1995) also showed nar- by this evening, so can't rule out thunder, even in the row radar bands of signi®cant surface icing and men- freezing rain areas near Fort Smith.'' The Little Rock tioned observations of thunder. The linear lightning NWSFO forecast discussion at 2130 CST 15 Janu- paths on 16±17 January have similarities to thunder- ary/0330 UTC 16 January pointed out the need to storms across the south central United States associated ``use some intuitive forecasting skills based on ex- with frozen precipitation in Colman (1990a,b). Those perience and interpretation'' concerning the type and thunderstorms originated as upright convection in amount of precipitation that would occur as numer- warm air south of the warm front and averaged 200 km ical models predicted southerly component air to in length for 4.9 h on radar. override the arctic air. Little Rock issued accurate Quasigeostrophic analyses at 0000 UTC 17 January winter storm watches at this time across the northern are in Fig. 15. Temperature advection has a contribu- half of the state; winter storm warnings 7 h later tion to upward vertical motion from central Texas stated that ``signi®cant ice accumulations are possi- northeast into eastern Oklahoma in a similar orientation ble.'' At 0430 CST/1030 UTC 16 January, Little as lightning at this time (Fig. 9b). There is also a con- Rock issued winter storm warnings over north and tribution to upward motion from differential vorticity central Arkansas for storm total accumulations of 10 advection in northeast Texas. CSI was indicated to be cm (4 in.) of ice and snow. Arkansas forecast prod- present in this region in Fig. 14. ucts ®rst mentioned lightning on the evening of 16 January in a forecast of a chance of thunder in above- e. Forecast issues freezing air over the southern part of the state. At 0915 CST/1515 UTC 16 January, Little Rock noted The NWS National Severe Storms Forecast Center that radar echo coverage ``had increased considera- included the possibility of thunderstorms associated bly over the previous couple of hours.''

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ing at 0200 UTC. At 850 mb, the eta model was 1ЊC too cold 12 h before the lightning event (Table 2), 3ЊC warmer than observed when lightning occurred at 0000 UTC 17 January, and continued 3ЊC too warm 12 h later. The model forecast warming and vertical motions above the surface quite well. But its output at the surface indicated too large an area of above-freezing temperatures in northern Arkansas than actually occurred, so the conclusion from the model is rain rather than freezing rain and sleet in that area. Cortinas and Crisp (1995) describe some of the important factors to con- sider in numerical model forecasts of frozen precipitation.

5. Conclusions Network-detected CG lightning was associated with frozen precipitation across the south-central United States during two events. Lightning was compared with coincident radar and surface data, and synoptic- and subsynoptic-scale conditions were analyzed at times and places of ¯ashes. The lightning network on 10 January detected 27 CG ¯ashes scattered in western Missouri where surface temperatures were between 0Њ and 2ЊC. The network detected 2417 ¯ashes on 16±17 January; ®rst lightning was in regions below freezing at the ground, then new southwest±northeast lines started in areas above freezing and ended in subfreezing surface air. Thunder- storms were not observed at any surface stations on 10 January and reported at only two stations on 16±17 January. The ratio of positive ¯ashes to total was 59% on 10 January and 52% during the ®rst 4 h on 16±17 January in a region with subfreezing surface temperatures. For the entire 16±17 January case, the positive FIG. 15. As in Fig. 8 except at 0000 UTC on 17 January 1994. ratio was 29%. WSR-88D re¯ectivity cores reached moderate intensity at the same time as lightning began in both cases. Areal coverage of echoes became extensive at the same A 48-h time series of eta model forecasts for a point time as ®rst lightning in one event, and several hours 30 km southeast of Fort Smith is shown in Fig. 16. earlier than ®rst ¯ashes in the other. Echoes in the vi- Results are from the model analysis package cinity of lightning had tops up to 6 km in one case and PCGRIDDS developed for operations by the NWS us- 10 km in the other. Some re¯ectivity levels were strong ing mandatory pressure level data. The shaded above- and in partial concentric rings because of beam inter- freezing layer from 760 mb to the surface is forecast to section with the bright band. Because of the melting last about 24 h starting at 1200 UTC 16 January (Fig. level, it was dif®cult to locate stronger storms using 16, top). At the time of lightning at Fort Smith (0000 radar re¯ectivity alone, as also occurs during other UTC 17 January, labeled as 1200 in bottom panel), cold-season situations without thunderstorms. Light- upward motion is predicted in a deep layer extending ning with echo tops identi®ed downstream regions with above 300 mb. Surface winds were forecast by the a potential for frozen precipitation. model to be southeast at the location near Fort Smith Layers between 800 and 900 mb had strong moist (Fig. 16), but observed winds were northeast (Fig. southwest ¯ow and were warmer than freezing in the 10). Surface temperatures were forecast by the model lightning regions for both events. Lightning occurred to be above freezing for more than 12 h near Fort Smith in a CSI region on 16±17 January but not on 10 Jan- (Fig. 16, top), but actual temperatures were at or below uary. Quasigeostrophic analyses showed that 850-mb freezing over the northern third of Arkansas, except temperature advection and the 850±500-mb differen- Fort Smith reached 0.6ЊC(33ЊF) for a few hours start- tial vorticity advection were similar in magnitude.

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FIG. 16. Time sections of eta model forecasts with 12-h resolution at 35ЊN, 94ЊW (30 km southeast of Fort Smith) from 1200 UTC 16 January (right) through 1200 UTC 18 January (left). Abscissa is from time of model run (0) on right through 48 h later on left (4800). Pressure (vertical axis) is labeled in mb. (Top) Temperature shown by solid contours in ЊC; regions warmer than 0ЊC are shaded. (Bottom) Vertical velocity (mbs01) shown by dashed contours is upward motion, downward motion shown by solid contours. Relative humidity above 70% only is contoured and shaded; contours are solid and labeled in tens of percent. Wind barbs as in Fig. 6.

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