Flash Flood Forecasting: The Probabilistic Prediction of Excessive Precipitation by the Hydrometeorological Prediction Center

The Hydrometeorological Prediction Center (HPC) issues an operational Excessive Rainfall Forecast several times each day. After many years as a deterministic product, the Excessive Rainfall Forecast was modified significantly in October 2004 to become a probabilistic forecast. This change was needed to advise users better of the threat for excessive rainfall, associated flash flooding, and meet NWS requirements to provide "weather, water, and climate forecasts in probabilistic terms." The new product provides better consistency with HPC's probabilistic heavy snow forecasts and the categorical convective outlooks of the Storm Prediction Center. The previous forecasts were a collection of disjointed indicators of areal coverage and probability without a clear increasing or decreasing threat scale. The current product starts with a slight (20-39%) threat increasing to moderate (40-69%) and high (70% or greater) of exceeding the flash flood guidance.

Preparation of the Excessive Rainfall Forecast starts with a detailed investigation and application by HPC forecasters of the numerical guidance, pattern recognition, model biases, model trends, and terrain adjustments with the result being a Quantitative Precipitation Forecast (QPF) for the conterminous U.S. (CONUS). HPC forecasters then compare this QPF to the River Forecast Center (RFC) Flash Flood Guidance (FFG) by county to identify areas where 1-h, 3-h, and 6-h FFG might be exceeded. Where needed the forecaster then assigns a probabilistic threat value to portions of the CONUS. The final product is used extensively by NOAA NWS Weather Forecast Offices (WFOs), RFCs, and federal, state, and local emergency managers in their daily forecasts, weather briefings, preparations, and other activities.

In addition to describing HPC's Excessive Rainfall Forecasts this paper will also discuss plans for the provision of QPFs as part of the expansion of NWS products and services in the digital era.