1. The U.S. Desert Southwest (and Las Vegas) Flash Floods, 7-11 July 1999 Matthew Kelsch Hydrometeorologist UCAR/COMET ® (Cooperative Program for Operational Meteorology, Education and Training) Boulder, Colorado USA kelsch@comet.ucar.edu

2. The U.S. Desert Southwest (and Las Vegas) Flash Floods, 7-11 July 1999 Terrain features focus initiation and regeneration, increasing rainfall duration Atypical maritime-tropical characteristics increased rainfall efficiency and rates Desert soils and drainages are very fast responding (more rapid runoff) Urbanization increased runoff complexity

3. 8 July 1999 18z: 500 mb height (ETA) and water vapor imagery

4. 1200 UTC 8 July 1999: 850 mb wind (knots) and dewpoint with precipitable water image (inches).

5. 1200 UTC 8 July 1999 surface analysis

6. Desert Southwest Topography and Radar Accumulation (inches) for 8 July 1999

11. The U.S. Desert Southwest (and Las Vegas) Flash Floods, 7-11 July 1999 Arid areas are very prone to flash floods from short-duration, intense storms: Rapid response on small (normally dry) basins Urban development in flood hazard areas