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

Abstract

Severe flash flood storms that occurred in Las Vegas, Nevada, on 8 July 1999, were unusual for the semiarid southwest United States because of their extreme intensity and the morning occurrence of heavy convective rainfall. This event was simulated using the high-resolution Regional Atmospheric Modeling System (RAMS), and convective rainfall, storm cell processes, and thermodynamics were evaluated using Geostationary Operational Environmental Satellite (GOES) imagery and a variety of other observations. The simulation agreed reasonably well with the observations in a large-scale sense, but errors at small scales were significant. The storm's peak rainfalls were overestimated and had a 3-h timing delay. The primary forcing mechanism for storms in the simulation was clearly daytime surface heating along mountain slopes, and the actual trigger mechanism causing the morning convection, an outflow from nighttime storms to the northeast of Las Vegas, was not captured accurately. All simulated convective cells initiated over and propagated along mountain slopes; however, cloud images and observed rainfall cell tracks showed that several important storm cells developed over low-elevation areas of the Las Vegas valley, where a layer of fairly substantial convective inhibition persisted above the boundary layer in the simulation. The small-scale errors in timing, location, rain amounts, and characteristics of cell propagation would seriously affect the accuracy of streamflow forecasts if the RAMS simulated rainfall were used in hydrologic models. It remains to be seen if explicit storm-scale simulations can be improved to the point where they can drive operationally useful streamflow predictions for the semiarid southwest United States.