a contrast of two las vegas flash flood scenarios introduction/backgroundintroduction/background a...
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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