Meteorological Characteristics of Flash Flood Events over the Western United States

Abstract

Meteorological characteristics of flash floods and heavy precipitation events over the western United States are often considerably different than those associated with flash floods in the east. Data were compiled and studied for 61 western United States flash flood events and attendant meteorological conditions were documented. The events were classified by similar 500 mb flow regimes and four characteristic patterns were associated with the flash flood events. Synoptic storms occurred in conjunction with both an intense cyclonic surface system and a strong 500 mb trough. The remaining flash floods (49 events) were associated with weak and slow moving 500 mb short-wave troughs. It is shown that the National Meteorological Center's broadscale analyses (surface and standard level charts) are not sufficient for monitoring the subtle features that often interact to produce flash floods. The field forecaster must enhance, or reanalyze, these facsimile charts if he is to identify regions of developing flash flood potential. Distinct geographical. seasonal and diurnal characteristics were noted for the four basic types of western flash floods and mean values of important meteorological parameters were computed.