The July 8, 1999 Las Vegas Flash Flood. The Monsoon Season in Las Vegas.
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Transcript of The July 8, 1999 Las Vegas Flash Flood. The Monsoon Season in Las Vegas.
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The July 8, 1999 Las Vegas Flash The July 8, 1999 Las Vegas Flash FloodFlood
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The Monsoon Season in Las Vegas
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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
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Composite Sounding for 8 LVCZ EventsCAPE=625 J kg-1 Mean 1-4 km wind ~ 230/06 ms-1
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Typical Las Vegas Area Downburst
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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
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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?
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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:
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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
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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
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