A Numerical Investigation of Storm Structure and Evolution during the July 1999
Las Vegas Flash Flood in Summary

Numerical predictions of intense convective events were generally unreliable and specific cases would have to be examined in order to make possible improvements to forecasts. Forecasts were especially inaccurate when storms were within areas of weak synoptic flow, as seen on July 8th, 1999 in the Las Vegas, Nevada flash flood storms. Researchers worked on this issue by constructing a simulation of the Las Vegas event using a fine-resolution Regional Atmospheric Modeling System (RAMS) model. They compared this simulation to the actual event and to the high-resolution prediction with an objective of modifying future numerical predictions of the Colorado Plateau area.
The semiarid Las Vegas, Nevada region is known for its diverse topography and summertime subtropical weather systems. On July 8th, 1999, Las Vegas experienced storm systems that were particularly abnormal and produced unusually severe flash flooding. Development occurred in the middle troposphere ahead of an inverted trough moving westward from south-central Arizona. Precipitable moisture amounts reached 40 mm and convective available potential energy (CAPE) values exceeded 1000 J kg-1. Observers noted low inhibition to convection and morning storm development. Rainfall rates exceeding 75 mm led to flash flooding, which resulted in two deaths, $20 million in property damage, and rapid rise in water levels and flow rates.
Conclusions were drawn using Geostationary Operational Environmental Satellite (GOES) infrared imagery. Advances in microphysics relating to mesoscale convective system (MSC) processes were also utilized. The RAMS Version 4.2 scheme categorized atmospheric water in order to determine mixing ratio. Additionally, upper-air soundings and hydrologic surface observations were taken into consideration.
It was determined that RAMS fine-resolution model simulation was more reliable than the Eta Model, as it slightly