A Contrast of Two Las Vegas

Flash Flood Scenarios• Introduction/BackgroundIntroduction/Background• A Classic Monsoon CaseA Classic Monsoon Case

»July 8, 1999July 8, 1999

• A Transition Flow CaseA Transition Flow Case»August 10, 1997August 10, 1997

The Monsoon Season in Las Vegas

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)

• 3.55” (7/3/01)Searchlight Pass

• 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

At McCarran: Within Clark County:

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

Classic Flash Flood SignaturesIllustrative Case: July 8, 1999

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)

• storm-relative inflow of buoyant air may be as important as cold pool-shear balance… but usually difficult to assess accurately

Forecasting Problems

• DRA often not representative of LV valley• model soundings typically not very valuable• convective structure/evolution sometimes

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?

Transition Flow SignaturesIllustrative Case: August 10, 1997

DRA Sounding – 1200 UTC 10 August 1997 CAPE=654 J kg-1 Deep-layered Shear ~ 40 kt PW=27 mm

12Z Eta 00h 310K theta surface & mixing ratio

GOES-9 Visible Image: 14Z - 10 Aug 97

GOES-9 IR Image: 1900 UTC - 10 August 1997

GOES-9 IR Image: 2100 UTC - 10 August 1997

GOES-9 Sounder CAPE: 2000 UTC – 10 Aug 97

GOES-9 Sounder LI: 2000 UTC – 10 Aug 97

KESX WSR-88D Base Velocity - 10/1922Z

Schematic of System Propagation

Composite Reflectivity: 10/2020Z

VAD Wind Profile: 10/1922-2020Z

Composite Reflectivity: 11/0047Z

VAD Wind Profile: 11/0024-0122Z

Storm Total Precipitation – 10 August 1997

Conclusions• 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

• Interplay between meteorology and hydrology can substantially influence a storm’s severity