The Relationship between intense, short-duration precipiation and flash floods

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 are often 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. The complex interrelationships between atmospheric moisture, low level features, and orography can result in storm-specific enhancement of precipitation efficiency. But flash flooding is more than a precipitation forecasting problem. The occurrence of significant flash floods is not directly correlated to rainfall amounts, and some major flash floods occur with unremarkable rainfall accumulation. Basins that respond very rapidly can experience significant flash flooding with intense, short-duration precipitation rates. Severe weather followed by flash flooding is an increasingly common hydrologic problem in rapidly responding urban watersheds. Thus, the hydrology plays a more critical role than precipitation amounts in some flash flood events. Improvements in flash flood forecasting will 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 represent landfalling hurricanes, terrain-locked storms, mesohigh storms, urban flash floods, West Coast heavy rain, and the role of forest fires on forest hydrology.