FLASH-FLOOD FORECASTING - AN INGREDIENTS-BASED METHODOLOGY

An approach to forecasting the potential for flash hood-producing stor ms is developed, using the notion of basic ingredients. Heavy precipit ation is the result of sustained high rainfall rates. In turn, high ra infall rates involve the rapid ascent of air containing substantial wa ter vapor and also depend on the precipitation efficiency. The duratio n of an event is associated with its speed of movement and the size of the system causing the event along the direction of system movement. This leads naturally to a consideration of the meteorological processe s by which these basic ingredients are brought together. A description of those processes and of the types of heavy precipitation-producing storms suggests some of the variety of ways in which heavy precipitati on occurs. Since the right mixture of these ingredients can he found i n a wide variety of synoptic and mesoscale situations, it is necessary to know which of the ingredients is critical in any given case. By kn owing which of the ingredients is most important in any given case, fo recasters can concentrate on recognition of the developing heavy preci pitation potential as meteorological processes operate. This also help s with the recognition of heavy rain events as they occur, a challengi ng problem if the potential for such events has not been anticipated. Three brief case examples are presented to illustrate the procedure as it might he applied in operations. The cases are geographically diver se and even illustrate how a nonconvective heavy precipitation event f its within this methodology. The concept of ingredients-based forecast ing is discussed as it might apply to a broader spectrum of forecast e vents than just Bash hood forecasting.