The July 8, 1999 Las Vegas Flash The July 8, 1999 Las Vegas Flash FloodFlood

The Monsoon Season in Las Vegas

Southern Nevada Thunderstorm Days(average morning sounding parameters)

• deep, well-mixed elevated boundary layer• 700-500mb lapse rate > 7 C km-1

• surface-700mb theta-w > 17 C (mean mxr > 8 g kg-1)

• average 12Z CAPE only about 250-300 J kg-1

• modest deep-layer (0-6km) shear• propagation into valleys dependent on:

• mean wind in the cloud-bearing layer• ambient vertical wind shear• bouyancy of the surface inflow layer

Composite Sounding for 8 LVCZ EventsCAPE=625 J kg-1 Mean 1-4 km wind ~ 230/06 ms-1

Typical Las Vegas Area Downburst

Monsoon Regime Challenges• continual fluctuation between subtropical

easterlies and polar westerlies• poor sampling of short waves in easterlies• relatively poor density of surface data• typically low-shear environment (therefore,

the primary ingredient = thermodynamics)• DRA sounding frequently not representative

of conditions in the Las Vegas valley

Monsoon Regime Challenges• model soundings typically not very valuable

(boundary layer modeled poorly in the west)• convective structure and evolution is often

modulated by local circulations • what buoyancy/shear values signal potential

for organized convection vs. isolated storms?• how can forecasters assess the influence of

storm-relative inflow and internal feedback processes which alter the ambient conditions?

Exceptional Storm Totals

• 2.59” (8/21/57)• 1.75” (8/10/42)• 1.56” (8/12/79)• 1.36” (7/28/84)• 1.34” (8/17/77)• 1.32” (7/24/56)• 1.29” (7/24/55)• 1.25” (7/26/76)

• 3.19” (7/8/99) Blue Diamond Ridge

• 3.13” (8/10/97) Boulder City

• 2.24” (9/11/98) Meadow Valley Wash

• 2.05” (7/19/98) Flamingo Wash

• 1.89” (9/11/98) California Wash

At McCarran: Within Clark County:

Concluding Remarks• The frequency of significant flash floods in

Las Vegas is higher than climatology suggests• As the metro area expands, the impact of such

storms will continue to increase• Interplay between meteorology and hydrology

can substantially influence a storm’s severity• Most flash floods are not characterized by the

classic signatures displayed in the July 8 storm

Forecasting Challenges• Accurate assessment of severe/flash flood

potential requires understanding of processes which influence convective structure

• relationship between buoyancy and shear• maintenance of unstable storm-relative inflow

• The mode of convection frequently changes during the course of an event.

• impact of local changes in stability, shear, lifting, etc.• interdependence of relatively large scale observable

trends with complex, meso/storm scale circulations