POST-ANALYSIS OFTHE 10 AUGUST 1997 SOUTHERN NEVADA FLASH FLOOD EVENT

Kim J. Runk and David P. Kosier

NOAA/National Weather Service Office Las Vegas, Nevada

Abstract

While both flash floods and severe thunderstorms occur each summer in southern Nevada, the storm of 10 August 1997 was exceptional. Heavy convective rains recurring over the same local area for several hours resulted in a 100-year flash flood, according to a report by the Clark County hydrologist. Flood damage was further aggravated by a period of thunderstorm-related winds exceeding 80 mph. Thermodynamic profiles were quite unstable owing to the influence of the moist "Mexican monsoon" regime. However, strong westerlies aloft impinging on this environment added an element atypical of monsoon conditions. Proximity soundings suggested u1}usually large CAPE with substantial vertical wind shear, favoring retriggering of storms and potential mesoscale convective system (MCS) development. The evolution of the convective system is of particular interest for at least two reasons. First, this case represents an outstanding example of assembling the ingre­dients for a slow-moving MCS within a large-scale envi­ronment characterized by moderate-to-strong tropospheric winds. Second, the mode of convection changed character in concert with the interaction of the synoptic scale jetstreak and other key mesoscale features including convective out­flow boundaries, an apparent low-level wind maximum, and local topography. The storm's severity was clearly impacted by the complex interplay among these meteoro­logical and hydrological factors. Understanding the impor­tant processes governing convective structure and organi­zation is crucial to successfully anticipating and accurately forecasting extraordinary events such as these.

1. Introduction

On Sunday, 10 August 1997, severe thunderstorms accompanied by significant flash flooding produced wide­spread and costly damage across the Henderson and Boulder City areas of extreme southern Nevada. Public facilities in Boulder City sustained an estimated $1.S5 million in damages with an additional $350,000 in dam­ages to uninsured property. In Henderson, the cost of storm-related destruction totaled nearly $1.5 million. Tragically, one fatality also resulted from these storms. The Boulder City Public Works Department reported that many of the city's streets were impassable during the rainfall, and some suffered loss of pavement as a result of the undercutting of flood waters. Three auto­mated rain gauges recorded two to three inches of rain-

10

fall in less than one hour. The steep desert landscape which characterizes this area simply could not mitigate the effects of such a downpour. According to the Clark County Regional Flood Control District (Sutko 1997), sev­erallocations met or exceeded the 100-year flood event criterion. In Boulder City, the impact of the flooding was exacerbated by thunderstorm-driven winds estimated at SO-90 miles per hour, resulting in at least 40 mature trees being snapped in two. This paper will describe the synop­tic conditions leading to the development of this event, detail the evolution of several key mesoscale features, then draw some conclusions regarding important rela­tionships between the meteorological and hydrological factors which influenced the system's severity.

2. Synoptic Overview

Analysis of upper air data, valid 1200 UTC 10 August 1997, indicated a mid-tropospheric trough extending from western Montana through a closed low over central Oregon, then offshore through northern California (Fig. 1). Satellite cloud-drift winds and offshore aircraft reports (not shown) depicted a jetstreak of nearly 40 m S-1 approaching the southern California coast. These reports were substantiated by a 33 m S-1 westerly wind at 250 mb on the Vandenberg, California radiosonde obser­vation (Fig. 2). Over southern Nevada, modest cooling had taken place in the 300-500 mb layer during the pre­ceding 24-hour period (Fig. 3). At the same time, sub­stantial moistening had occurred in the lower levels, as evidenced by an average increase in dewpoint of 4-7 °C below 700 mb. In addition, surface dewpoints exceeding 15°C were common across the area, and clear skies were expected to allow several hours of unimpeded boundary layer heating, leading to very large lapse rates by early afternoon. Short term extrapolation suggested the devel­opment of atypical summer soundings for the area with CAPE values exceeding 2000 J kg1 and pronounced ver­tical wind shear of - 24 m S-1 through the 0-6 km layer (Figs. 4 and 5).

The influence of strong westerlies impinging on a lower troposphere rich in moisture produced an environ­ment of potential instability conducive to very high rain rates as defined by R = Ewq. R is the Rainfall Rate, E is the Precipitation Efficiency coefficient (a ratio of the mass of water falling as precipitation to the mass of cloud water influx), w is the ascent rate, and q is the mixing ratio of the rising air (Doswell, Brooks and Maddox 1996).

Volume 22 Number 4 December 1998

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Volume 22 Number 4 December 1998

3. Mesoscale Aspects of the Convective Environment

Initial thunderstorm development focused on a quasi­stationary convergence boundary oriented east to west, which had been created by outflow from the previous evenings convective activity (Fig. 6). A small mesoscale convective system (MCS) had persisted through the night over extreme southwest Utah. Between the hours of 1600 and 1800 UTC, animated satellite imagery and radar data revealed an organized outflow boundary, originating from this MCS, being ejected toward the southwest. By 1900 UTC (noon PDT), new cell growth was observed to form at the intersection of this secondary outflow boundary and the western end of the pre-existing convergence zone, which was anchored to high terrain in the McCollough Range west of Lake Mead (Fig. 7). This feature seemed to be the mechanism by which convective updrafts were ini­tially lifted to the level of free convection (LFC).

Additional forcing associated with jetstreak dynamics apparently contributed to the development of organized deep convection along this interface by early afternoon. Satellite animation depicted explosive growth coincident with the approach of the upper-level jet maximum (Fig. 8). While the source of lift was not located beneath the traditionally favored left exit region with respect to the jet maximum, it has been shown that substantial forcing exists around the entire nose of a cyclonically curved jet­streak (Moore and Van Knowe 1992), consistent with this feature's orientation.

Boundary layer moisture had been on the increase for several hours prior to the initiation of deep convection, owing to a strong southerly flow from the Gulf of

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Fig. 5. Desert Rock hodograph, valid 1200 UTe 10 August 1997 (scale = m s-'; hodograph tick marks every 1000 m above mean sea level)

evolution ofthe low-level moisture field implies that con­tribution. Numerical guidance from the NOAAlNational Weather Service/ National Centers for Environmental Prediction (NCEP), 29-kro version Eta model predicted 800 mb dewpoints of around 12°C with substantial mois­ture flux convergence developing in the area by 1800 UTC (Fig. 10). The Eta OO-h 310 OK theta surface depicts the low-level moist tongue well (Fig. 11). It is understood that diabatic heating in this region affects the isentropic

California up the Colorado River valley in the surface to 850 mb layer. Surface dewpoints climbed to near 20°C by 1800 UTC. The rapid increase in moisture was likely a combination of quasi-hor­izontal advection and downward vertical mixing of elevated mois­ture as the boundary layer deep­ened with heating. Similar histor­ical cases have been documented in which an elevated wind maxi­mum of about 15 m S·I served to transport moist air rapidly north­ward above the morning inversion (Douglas 1995; Runk 1996). Observations to verify the exis­tence of such a nocturnal jet on 10 August 1997 were unavailable. However, a southerly low-level wind field of approximately 10-15 m S-I extending from the Gulf of California through the lower Colorado River valley was pro­duced in a post-event modeling simulation using a nonhydrostat­ic version ofthe Aster Corporation RAMS model (Pielke et al. 1992) at lO-kro horizontal grid spacing (Fig. 9). Moreover, the temporal

Fig. 6. GOES-9 visible image, valid 1400 UTe 10 August 1997 (convective outflow boundary indicated by arrow)

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14 National Weather Digest

Fig. 7. GOES-9 infrared image, valid 1900 UTC 10 August 1997 (thunderstorms forming at intersection of secondary outflow boundary and pre-existing convergence zone)

Fig. 8. GOES-9 infrared image, valid 2100 UTC 10 August 1997 (dashed lines = 200 mb isotachs in kt from NCEP/RUC 6-h forecast, valid 2100 UTC 10 August 1997)

Volume 22 Number 4 December 1998

sunaces well above this layer, but since this analysis is valid for 1200 UTC (0500 PDT), it is presumed to be fairly represen­tative of the large scale character of mois­ture advection into the pre-storm environ­ment.

This warm, moist southerly flow, in con­cert with intense sun ace heating, contin­ued to destabilize the air mass as suggest­ed by the 850 mb theta-e forecast (Fig. 12), and acted to erode the weak capping layer. Comparison ofthe 1200 UTC Desert Rock soundings from 9 and 10 August 1997 shows low-level moisture had deep­ened to a layer nearly 200 mb thick, with 1.04 inches of precipitable water and a mean wet-bulb potential temperature in the sunace to 700 mb layer of 21.8 ec. The accompanying hodograph exhibits signifi­cant clockwise curvature with vertical wind shear of - 24 m S·l in the lower 6 km AGL, and a Bulk Richardson number of 14, suggesting the potential for long-lived rotating updrafts (Weisman and Klemp 1984; Weisman and Klemp 1982; Rotunno 1981). Moreover, the wind profile in the lower 3 km AGL displayed sufficient shear to support organized multicell evo­lution in the downshear direction, leading

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National Weather Digest

to possible MCS development, given a lin­ear trigger mechanism. Conservative modification ofthe morning sounding sug­gested afternoon CAPE values between 1500-2500 J kg\ implying large ascent rates (w) and large input mixing ratios (q): fundamental factors in the production of high rainfall rates as defined by the equa­tion in Section 2. Indeed, by 2000 UTC, the GOES sounder data indicated CAPE values of about 1800 J kg! (Fig. 13) and lifted index values in the -5 to -7°C range (Fig. 14).

During the early stages of system development, storms continued to form along the convergence zone, then move northeast with the mean wind. Reflectivity and velocity data from the KESX (Las Vegas area) WSR-88D radar, as well as visual observations from trained spotters, indicated new cell growth was recurring near the downdraft of preceding storms (Fig. 15). This config­uration, suggestive that gust front processes were dominating the retrigger­ing of storms, persisted for about an hour. Given the relatively high environmental humidity, it is assumed that evaporation rates were low enough such that the cold

Fig. 12. Eta-48 12-h forecast of 850 mb theta-e (OK) and wind (full barb = 10 kt), valid 0000 UTe 11 August 1997

Volume 22 Number 4 December 1998

pool of a decaying cell was not strong enough to disrupt or undercut the moist boundary­relative inflow to new updrafts along the southern flank of the convergence line. Subsequent cells could then track northeast with the mean wind along a tra­jectory similar to their predeces­sors. This motion reinforced the boundary and maintained its position relative to the low-level moisture axis.

Following the schematic of Chappell (1986), the cell motion vector was directed toward the northeast, while the propagation vector was oriented toward the southwest (though with a small­er magnitude), resulting in a system motion vector which was nearly stationary (Fig. 16). From an Eulerian viewpoint, east Henderson and Boulder City were located in a region experi­encing repeated convective rains from multiple cells for: a periQd of several hours. Thus, the ingre­dients were in place for a slow­moving MCS to form within a large-scale environment charac­terized by moderate-to-strong tropospheric winds.

4. Evolution of Storm Structure

By 2020 UTC, thunderstorms had organized into a solid line oriented northeast to southwest from the Lake Mead National Recreation Area through Boulder City and Henderson to the McCollough Range (Fig. 17). Elevated cores of high reflectivi­ty were noted as storms approached Boulder City. Enhanced moisture convergence may have contributed to this intensification as mesonet winds depicted a small but distinct cyclonic circulation in the wind field southwest of Boulder City with well-defined confluence in its northeast quadrant (Fig. 18).

Immediately following this rapid intensification, the strongest reflectivity core was observed to split, with the most intense cell moving to the right of the mean storm motion vector (Fig. 19). This cell displayed

Fig. 13. GOES-9 sounder CAPE (J kg-'), valid 2000 UTC 10 August 1997

marked rotation and triggered Fig. 14. GOES-9 sounder Lifted Index (0C), valid 2000 UTC 10 August 1997

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the WSR-88D mesocyclone algorithm at 2101 UTC (Fig. 20). These attributes are characteristic of supercell evolu­tion (Lemon and Doswell 1979). Reports of winds estimat­ed at 80-90 mph and hail up to one-inch in diameter were associated with this thunderstorm. A storm survey revealed widespread wind damage along the storm's path, including dozens of mature trees downed or uprooted in Boulder City. The coincidence of this process with the pas­sage of the upper-level jetstreak suggests the zone of

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boundary layer convergence (and associated vertical wind shear) was being strengthened and organized by larger scale quasi-geostrophic forcing. Most notably, the layer between 4-6 km above the surface (represented on Fig. 21 as 20-26 kft above mean sea level or 15-21 kft above radar level), sustained an increase in wind speed from about 15 m S·l at 1922 UTC to over 25 m S·l at 2020 UTC.

As the influence oflarger scale dynamic processes and attendant vertical wind shear increased, the character of

the convective structure evolved first into a line echo wave pat­tern (Fig. 22), then into two small bowed echoes (Fig. 23), which accelerated northeast into Arizona. The V AD Wind Profile for this time (Fig. 24) indicated moderate wind shear in the low- . est 2000-3000 ft above radar level, with generally steady winds through the mid-tropos-phere. Although the bow echoes were neither classic nor intense,

Fig. 16. Schematic of system propagation, valid 2000-2300 UTC 10 August 1997 (C = cell motion vector, P = propagation vector, S = system vector)

the conditions within which they developed were consistent with those favoring bow echo forma-

Volume 22 Number 4 December 1998 19

tion in numerical simulations documented by Weisman (1993) and confirmed observa­tionally by Johns (1993). In particular, these studies sug­gest optimum wind conditions for development and mainte­nance of bow echoes consist of moderate-to-strong vertical shear in the lower 1-2 km AGL, with uniform winds above. Albeit marginal in both depth and strength, the prox­imity wind shear profile falls within these parameters dur­ing the time in which bow structures formed on the afternoon of 10 August 1997.

In summary, the changing nature of the thermodynamics and kinematics, coupled with increasingly complex convec­tive scale interactions, result­ed in at least three different stages in the mode and orga­nization of the deep convection on 10 August 1997 (multicel"~ lular line, isolated mini-super­cells, bow echoes).

5. Hydrological Factors

The process of many cells passing over the same area in rapid succession (the so-called "train echo effect") is arguably the most dangerous of situa­tions for producing life-threat­ening floods in many areas of the country, the southwest US included. However, in areas of complex topography, the inter­action of the meteorology with the specific character of the local terrain plays a crucial role in determining whether a meteorologically modest event might become a locally severe one from a hydrology perspec­tive.

Boulder City is located on the shelf of a broad alluvial fan which drains southward into the EI Dorado valley. This valley is bounded to the east by the EI Dorado Mountains and to the west by the McCollough Range (Fig. 25). The former is a small range oriented north-south parallel­ing the west bank of the Colorado River. The EI Dorado

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National Weather Digest

Mountains extend from just southeast of Boulder City to near the small town of Searchlight. The highest peak in the El Dorado Range is about 5000 feet above mean sea level (MSL). By contrast, the valley floor, only seven miles to the west, is approxi­mately 1900 feet MSL.

The city of Henderson is sit­uated along the foothills at the northern end of the McCollough Range. These mountains extend south­southwest to the Nevada­California border near the small town of Nipton, California. The highest peak in the McCollough Range is just over 7000 feet MSL. The dis­tance from the crest of the McCollough Range to the floor of the El Dorado valley is approximately ten statute miles.

Both the El Dorados and the McColloughs are composed largely of fractured volcanic rock, basalt, and alluvium with very low percolation capacity. Rainfall rates exceeding 0.50 inches in 30 minutes have con­sistently produced curb-high water flows through urban streets in Henderson and Boulder City. During the event of 10 August 97, one automat­ed gauge on a south-facing slope in Boulder City recorded 2.55 inches of rain in the 58-minute period between 1250 PDT and 1348 PDT. Furthermore, one-minute readouts suggest periods where the rain rate was too rapid for the tipping bucket mechanism to empty and reset in time to record accurately. Therefore, actual rainfall totals may have been higher than some of the automated reports indicated, perhaps on the order of 4-5 inches in the 3-hour period between 1300-1600 PDT. This is consistent with bucket gauge measure­ments taken in the area the following morning. An analysis of the storm total rainfall as recorded by automated sensors is depicted in Fig. 25.

Volume 22 Number 4 December 1998

6. Summary and Conclusions

Portions of extreme southern Nevada experi­enced significant flash flooding and damaging winds during the afternoon of 10 August 1997 due to recurring periods of heavy rain from mul­tiple convective cells. The slow-moving convec­tive system responsible for generating these rains developed within a large-scale environ­ment characterized by relatively strong mid-tro­pospheric winds. Furthermore, as the system evolved, the mode of convection was altered by local changes in buoyancy and vertical wind shear.

During the early stages ofthe system's devel­opment, new cell growth appeared to occur near the downdraft of preceding thunderstorms, based on radar velocity displays and visual observations. Given the high environmental rel­ative humidity, it is assumed that evaporation rates were low enough such that the cold pool of a decaying cell was not strong enough to disrupt or undercut the moist boundary-relative inflow to new updrafts. Moreover, the low LFC allowed

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Fig. 23. KESX WSR-88D composite reflectivity image, 0047 UTe 11 August 1997

for easy retriggering by gust fronts . Subsequent cells could then follow a trajectory similar to their predeces­sors. This motion thus reinforced the convergence bound­ary and maintained its position relative to the impinging low-level moisture axis, resulting in a linear, multicellu­lar structure dominated by gust front/cold pool processes.

Later in the system's life cycle, significant rotational components were observed, coupled with a storm track which deviated to the right of the mean wind. These attributes are suggestive of mini-super cell structure, and indeed, numerous reports of large hail and damaging winds coincided with this storm's passage through Boulder City. Several factors may have contributed to this development, including increased vertical wind shear associated with response to an approaching upper tropospheric jetstreak, and concentrated boundary layer vorticity being turned into the updraft.

In its final stages, the mode of convection was charac­terized by the development of a pair of bowed lines with evidence of rear inflow notches on the upshear flank.

During this period, the vertical wind shear became con­centrated largely in the lowest 2 km AGL with very little shear in the 2-6 km layer. While these structures were too small in scale and duration to be described as long-lived bow echo systems, the environment within which they formed is consistent with conditions known to be common in the development of similar configurations on the larg­er end of the bow echo spectrum.

Finally, the composition of the surrounding terrain exacerbated the effect of many convective cells passing over the same area in rapid succession. Steep topograph­ic grades composed largely of fractured volcanic rock and alluvium provided very little percolation capacity, result­ing in extremely high runoff and discharge rates. This interplay between the meteorology and hydrology multi­plied the damage associated with an already dangerous event.

From an operational forecaster's perspective, at least two useful lessons can be derived from this post-analysis. First, to successfully anticipate the potential for severe

Volume 22 Number 4 December 1998 23

thunderstorms on a given day, it is crucial to understand the processes which organize the structure of the convection, and in particular, the relation­ship between (and influence of) buoyancy and shear.

Second, it is important to recognize that the mode of convection frequently changes during the course of an event. Large scale forcing, moisture advection, interaction of boundary layer convergence zones, and the like, can trans­form an otherwise innocuous convective environment into a dangerous one by effecting local changes in static stabili­ty, vertical wind shear, lifting mechanisms, etc. Therefore,

. the formidable challenge of nowcasting involves continu­ally evaluating the interde­pendence of relatively large scale, observable trends in atmospheric variables with complex mesoscale and storm scale circulations. This is not a trivial task.

Notwithstanding the diffi­culty in accurately applying such knowledge within the time constraints of a real-time forecast, the fact remains that improved understanding of the general principles govern­ing convective evolution will undoubtedly result in more correct forecasts and warn­ings. In climatologically rare events (e.g., a 100-year flash flood) such an approach is cru­cial. Synoptic patterns associ­ated with such events, because they are so extraordi­nary, are often not recognized. If the continuing stream of technological advances is to be exploited effectively, it is imperative that training and education efforts at the local forecast office remain focused on acquiring the fundamental knowledge and skills intrinsic to improving the quality and reliability of forecasts.

AL T KFT

58 45

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23 ,,-/ V 'W" ,~v ,0/ .w: ,V &" .w ,0/ 'V

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~ "'. '" ...,.

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TIME 8824 8838 8835 0841 8847 8853 8859 0185 8111 8117 0122

04 / 09 / 98 131 : 1 {' UAD WIND PROFILE

48 UWP 88/11/97 01'22 RDA :K ES X 35 / 42 / 03N 4948 FT 114 / 53 / 27W

MODE A / 21

MA X=237 DEG 64 KT ALT 38000 FT

I t 16

KT RMS

FL= 1 COM=1

Q15 UAD 0188 R PROD RCUD SRM RPS KES X 0118 1 5 89/8114 *TIME OUT* CAN'T ED IT RCM HARDCOPY

ALERTS'

1 )GR2 ;' GR

Fig. 24. KESX WSR-88D VAD wind profile, 0024-0122 UTe 11 August 1997 (winds in kt, full barb = 10 kt; altitudes in kft MSL; radar level - 4900 ft MSL)

I.O"/} 1.0"/

Fig. 25. AnalYSis of storm total precipitation for 10 August 1997 in inches (based on rain gauge totals and bucket survey measurements synthesized with radar estimates)

I

24

Acknowledgments

The authors wish to express their appreciation to the following individuals for providing helpful editorial sug­gestions which significantly improved the manuscript: Dr. John Weaver, NOAAlNESDIS/ClRA; Dr. Doug Wesley, UCAR/COMET; Dr. John Snook, NOAAlERIJFSL; Ms. Wendy Schreiber-Abshire, UCAR/COMET; Mr. Ron McQueen, NWS Las Vegas. Thanks are also due to Dr. Weaver for providing the satellite image used in Fig. 6.

Authors

Kim Runk is the Science and Operations Officer at the National Weather Service office (NWSO) in Las Vegas, Nevada. He earned a B.S. degree in Atmospheric Sciences at Creighton University in 1988, then attended graduate school there from 1992-1994. Prior to joining the National Weather Service, Kim spent 20 years in operational meteorology with the U.S. Air Force. His experience ranges from producing combat mission forecasts to gen­erating national severe weather outlooks. Kim also spent several years with the USAF tropical cyclone analysis branch, providing support to both the National Hurricane Center and the Joint Typhoon Warning Center. He was on the combat weather team for the Grenada rescue mission (Operation Urgent Fury) in 1983, and acted as Chief Forecaster in the unit which developed planning forecasts in support of Operation Desert Storm in 1991. From 1992-1994, Kim was the Air Force representative on the COMET Advisory Panel.

Dave Kosier is a staff meteorologist with NWSO Las Vegas. He received a B.S. degree in Meteorology from Northern Illinois University in 1988. His National Weather Service career has provided opportunities to build experience in a variety of forecasting disciplines including aviation, public, marine, and fire weather. He has been assigned to NWS offices in Seattle, Fresno, and Las Vegas. Dave also enjoys international travel. Since 1995, he has visited Australia, New Zealand, Japan, Thailand, Singapore, and eight European countries.

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