1. A Contrast of Two Las Vegas Flash Flood Scenarios Introduction/BackgroundIntroduction/Background A Classic Monsoon CaseA Classic Monsoon Case »July 8, 1999 A Transition Flow CaseA Transition Flow Case »August 10, 1997

2. The Monsoon Season in Las Vegas

3. 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:

4. 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

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

6. Typical Las Vegas Area Downburst

7. Classic Flash Flood Signatures Illustrative Case: July 8, 1999

8. 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

24. 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?

25. Transition Flow Signatures Illustrative Case: August 10, 1997

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

28. 12Z Eta 00h 310K theta surface & mixing ratio

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

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

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

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

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

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

35. Schematic of System Propagation

36. Composite Reflectivity: 10/2020Z

37. VAD Wind Profile: 10/1922-2020Z

38. Composite Reflectivity: 11/0047Z

39. VAD Wind Profile: 11/0024-0122Z

40. Storm Total Precipitation – 10 August 1997

43. 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