A prototype operational 0-1 hour radar-based flash flood potential algorithm

Over the coming years, the National Weather Service will be implementing within AWIPS (Advanced Weather Interactive Processing System) an operational capability across the U.S. termed Flash Flood Monitoring and Prediction to monitor and predict flash flood potential utilizing radar-derived rainfall estimates and other information. The Hydrologic Research Laboratory has developed a prototype algorithm, called Flash Flood Potential (FFP), that compares WSR-88D rainfall estimates with "flash flood guidance" (FFG) rainfall that is computed by the NWS River Forecast Center's operational hydrologic models. These FFG rainfall amounts represent approximate threshold basin-average rainfall that would cause small streams to begin flooding.

The FFP computes gridded radar rainfall estimates for a single WSR-88D radar area and compares them with gridded FFG to assess the potential for flooding over small scale areas within the radar scanning domain over durations of one, three, and six hours. In addition, it produces a 0-1 hour forecast of rainfall based on the extrapolation of radar rainrate fields forward in time using a pattern matching technique. The forecasted rainfall is probabilistic in nature due to the uncertainty in the projection technique and the predictability of radar rainfall fields. The algorithm ultimately produces a probabilistic forecast of the likelihood that radar rainfall estimates will exceed FFG called the Critical Rainfall Probability.

The algorithm will be described, and its performance will be discussed. Two methods for computing the uncertainty associated with the forecasted rainfall will be compared by examining several case studies of heavy rain.