Flash Floods: Characteristics of the Precipitation, the Runoff, and the Social Response

Historically, the flash flood problem has been approached mainly as a QPF problem. Although accurate diagnoses and forecasts of mesoscale precipitation processes are very important, understanding the subsequent hydrologic response is equally as important and extremely variable depending on the basins involved. Obtaining appropriate actions by the public at risk is another important step in reducing flash flood impacts. However, the dangers of extreme rainfall and its consequences are often perceived as less dangerous than other forms of severe weather. Severe weather followed by flash flooding is an increasingly common hydrologic problem in rapidly responding urban watersheds.

Qualitatively, a flash flood can be described as a flood event in which the rainfall and the subsequent runoff are occurring on the same time and space scales. Intense precipitation rates may be more important than total accumulation, particularly when small, fast-response basins are involved. A crucial task in flash flood forecasting is analyzing and forecasting the ingredients leading to atypically intense precipitation rates or locally enhanced duration of convective precipitation. The complex interrelationships between atmospheric moisture, low level features, and orography can result in storm-specific enhancement of precipitation efficiency and the occurrence of successive precipitation bursts in a local area. However, hydrology can play a more critical role than precipitation amount in some flash flood events. The occurrence of significant flash floods is not directly correlated to rainfall amounts, and some major flash floods occur with relatively modest rainfall accumulation. Basins that respond very rapidly can experience significant flash flooding with intense, short-duration precipitation. Improvements in flash flood forecasting depend on advances in understanding and modeling storm-scale precipitation processes and the runoff character of fast-response basins.

The Cooperative Program for Operational Meteorology, Education and Training (COMET®) has been involved with the science of hydrometeorology for almost a decade. A diverse set of hydrometeorological case reviews are used to embellish the training material by demonstrating many of the common features, as well as crucial differences, among the events studied. The cases involve both historic rainfall events and non-historic events that resulted in major flooding. They demonstrate the impact of landfalling hurricanes, terrain-locked storms, mesohigh storms, urban flash floods, West Coast heavy rain, and altered hydrology due to forest fires. The various cases also demonstrate the crucial role of emergency response agencies and public awareness in affecting appropriate actions.