The July 8, 1999 Las Vegas Flash Flood

The Monsoon Season in Las Vegas

Southern Nevada Thunderstorm Days (average morning sounding parameters) deep, well-mixed elevated boundary layer mb lapse rate > 7 C km -1 surface-700mb theta-w > 17 C (mean mxr > 8 g kg -1 ) average 12Z CAPE only about 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 Events CAPE=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