NOAA Technical Memorandum NWS WR- 147 OCCURRENCE AND DISTRIBUTION OF FLASH FLOODS IN THE WESTERN REGION Thomas L. Dietrich Western Region Headquarters Hydrology Division Salt Lake City, Utah December 1979 UNITED STATES / NATIONAL OCEANIC AND / National Weather DEPARTMENT OF COMMERCE / ATMOSPHERIC ADMINISTRATION // Service Juanita M. Kreps, Secretary Richard A. Frank, Admtnistrator Richard E. Hallgren. Director This Technical Memorandum has been reviewed and is approved for pub! ication by Scientific Services Division, Western Region. L. W. Snellman, Chief Scientific Services Division Western Region Headquarters Salt Lake City, Utah ii TABLE OF CONTENTS Tables and Figures iv I. Introduction 1 II. Climatology of the Western Region by Geographical Divisions 1 a. Pacific Coast .. 1 b. Cascade ~ Sierra Nevada Mountains. 4 c. Intermountain Plateau. 4 d. Rocky Mountain Region. 4 e. Great Plains (eastern Montana) 8 III. Meteorology of Flash Floods .... 8 IV. Geographical Properties influencing Flash Floods. 10 a. Topography, Soils and Surface Cover. 10 b. Urbanization 10 V. Data Analysis .. 10 VI. Geographical Areas of Significant Concentrations of Flash Flood Events .... 17 a. Wasatch Front, Utah. 17 b. Central and Southern Arizona 19 c. Utah - Southern Mountains, Sevier Valley and Cedar City Area 19 VII. Brief Description of Flash Floods in the Western Region, 1950- 1969, as reported in the USGS Annual Flood Summaries. 23 VIII. References. iii TABLES AND FIGURES Table 1. Flash Floods by Geographical Region 16 Table 2. Flash Floods by Month and State . • 17 Figure 1. Geographic Regions of the United States 2 Figure 2. Coastal Valley, south of Eureka, California 3 Figure 3. Mean Annual Number of Days with Thunderstorms . 5 Figure 4. Cascade Range, Western Washington . • 6 Figure 5. Flood Debris lodged in Tree from Floodwaters in theEast Verde River, Arizona. 7 Figure 6. Heppner, Oregon . 9 Figure 7. Bitterroot Valley, near Hamilton, Montana 9 Figure 8. Little Cottonwood Canyon, near Salt Lake City, Utah 11 Figure 9. Big Cottonwood Creek, Salt Lake City, Utah. 12 Figure 10. Effects of Urbanizat~on of Flood Heights. 13 Figure 11. Flash Floods in the Western Region, 1950-1969, as reported in the U.S. Geological Survey Annual Flood Summaries • . • . • . 14 Figure 12. Flash Floods by Geographical Region 15 Figure 13. Monthly Distribution of Flash Flood Events as reported in the U.S. Geological Survey Annual Flood Summaries, 1950-1969 . 18 Figure 14. Wasatch Front, Salt Lake City, Utah 20 Figure 15. Central Arizona Mountains, near the Mogollon Rim. 21 Figure 16. Entrance to "The Narrows," Zion National Park, Utah 22 iv OCCURRENCE AND DISTRIBUTION OF FLASH FLOODS IN THE WESTERN REGION Thomas L. Dietrich Hydrology Division Salt Lake City, Utah I. INTRODUCTION Given the right meteorological conditions, a flash flood can occur almost anywhere in the Western Region. However, some areas are much more susceptible to flash floods than others. Of the eight states making up the Western Region, the two states reporting the greatest number of flash floods are Utah and Arizona. Nevada undoubtedly also has a high number of flash floods; however, they probably go unobserved because of the low population density. The purpose of this report is two-fold. First, it is to describe flash floods that have occurred in the Western Region as reported in the USGS Annual Flood Summaries from 1950-1969. At the same time, it is intended to provide the meteorologist and meteorological technician with a relatively geographical and seasonal distribution of flash flood e~ents in the Western Region. Data is taken only from the U. S. Geological Survey Annual Flood Summaries since it is not the intent to account for and describe every flash flood that occurred between 1950 and 1969. I am assuming that the sample is representative of the overall occurrence and distribution of flash floods in the Western Region. II. CLIMATOLOGY OF THE WESTERN REGION BY GEOGRAPHICAL REGIONS The conterminous United States can be divided into seven physical regions, five of which are represented in the Western Region (Figure 1). The principal climatic features of the geographical divisions of the Western Region are described below. a. Pacific Coast The Pacific Coast Region consists of the coastal plains, the coastal ranges, and the adjacent inland valleys of Washington, Oregon and California (Figure 2). ,_._,....___ __ ../Williston i ; Bismarc"k I • ;.____ ;I ~~iGityR i-----­ i ej ...._i i •N I San•AntoJ!io f.! EX L F 0 F SCALE 1:10,000.000 S.TI.N0-'11~ PAP~I.l..ELS Z91"io• ANO 451"io0 160" 136" ,... 115" 110" .... ... ... ... L Figure 1. Geographic Regions- of the United States (Baldwin, 1973) Figure 2. Coastal Valley, south of Eureka, California 3 With a Mediterranean type climate of dry summers and mild wet winters the region has the lowest mean annual number of days with thunderstorms in the United States (Figure 3). During the summer, the strong Pacific high steers storms to the north of the region. When the high weakens in winter, storms of greater areal coverage and intensity hit the coast more frequently. b. Cascade-Sierra Nevada Mountains This continuous chain of mountains (Figure 4) varies in width from 20 to 70 miles and is generally 8,000 to 10,000 feet high. It acts as a barrier to the free movement of air masses from both east and west. The region is dominated by the usually mild Pacific air brought in by the prevailing westerlies. Winter storms generally cover large areas, whereas the summers are generally fair, sunny and pleasant. c. Intermountain Plateau The Cascade-Sierra Nevada Ranges either modify the mild moist air masses reaching the region from the Pacific or prevents them from entering the region. To the east, the Rocky Mountains shield the region somewhat from incursions of warm moist air from the Gulf of Mexico and from the cold air that sweeps south from Canada onto the Great Plains. Summer rainfall in southeastern California,· Arizona, southern Nevada and southern Utah begins early in July and usually extends through mid-September. Moisture-bearing winds sweep into the area from the southeast, with their source region the Gulf of Mexico. Heavy thundershowers at times cause flash floods that do consider­ able local damage. This area also reported the greatest number of flash floods. The heaviest runoff usually occurs in connection with the arrival of tropical air over southern California, Arizona (Figure 5), southern Nevada and southern Utah, which had its origin in hurri­ canes that dissipated in or off the west coast of Mexico. Heavy rains associated with these systems usually occur during August or September. Figure 6 shows Heppner, Oregon sitting in the flood plain of Willow and Hinton Creeks where 200 people died in a flash flood in the early 1900's. d. Rocky Mountain Region The high ranges of the Rocky Mountains extend southward from Canada into northern, central, and eastern Idaho, western Montana - 4 - ( Vl I LEGEND DuNnER 20 § 20-40 M;Jrm 40 . so ·~ 60-80 11 8o-10o ,.,,.o II:<> ...,.A o<:rO I I G U L F I 0 F ALBERS EQUAl AREA PROJECTION I *! FSS THAN 1 IN 2 YEARS Figure 3. Mean Annual Number of Thunderstorm Days (Baldwin, 1973) Figure 4. Cascade Range, Western Washington - 6 - Figure 5. Flood Debris lodged in Tree from Floodwaters in the East Verde River, Arizona - 7 - (Figure 7), western and central Wyoming, northeastern Utah, western and central Colorado and north central New Mexico. Variations in precipitation occur over the rather wide range in latitude. Predominantly, the climate is associated with Pacific Ocean air masses which move inland from the west, carrying much of the region's precipitation. In the southern portion of the region the maximum monthly precipitation often occurs during July, August and September as a result of the summer thunderstorms that sweep in from the Gulf of Mexico. e. Great Plains (eastern Montana) Circulation around low-pressure areas moving toward the east or northeast across the central U. S. or across southern Canada is one of the causes of the northward flow of warm gulf air into eastern Montana. The wind around the western side of the Bermuda high­ pressure area intensifies in summer, contributing to the northward flow of moisture. III. METEOROLOGY OF FLASH FLOODS (Maddox and Chappel, 1978) Maddox and Chappel (1978) identified a number of meteorological para­ meters common to flash flood events in the western United States. Detailed analyses of three-hourly surface charts, standard level charts for 850, 700, 500 and 300 mb, hourly radar-summary charts and upper-air sounding data revealed that a number of large scale patterns, mesoscale features, and meteorological parameters were common to many events. For flash floods in the western U. S. the following features were found to be fairly common: 1) Storm area is very near large-scale ridge position. 2) A weak mid-level short-wave trough moving through or around ridge helps to trigger and focus storms. 3) Storms usually occur during the afternoon hours. 4) Rain amounts tend to be lighter than elsewhere in the U. S. 5) Severe thunderstorm phenomena are often associated with the heavy rainstorms. Generally western type events are confined to a region west of the continental divide and east of the Pacific Coast Ranges. The thunder­ storms responsible for the flooding quite often produce hail and high winds as well as heavy rain. Small-scale terrain features, local heating anomalies, and thunderstorm-scale motions interact in these western events to maximize rainfall in particular locations. - 8 - Figure 6. Heppner, Oregon Figure 7. Bitterroot Valley, near Hamilton, Montana - 9 - The general threat area can be best delineated by the high moisture contents (surface through at least 500mb), weak vertical wind-shear, and ;Large conditional instability that lies ahead of the advancing middle­ level short wave.