HPC's Excessive Rainfall Potential Outlook

The Hydrometeorological Prediction Center issues Excessive Rainfall Forecasts several times each day. This product experienced a major overhaul in October 2004, as the product became a purely probabilistic forecast. This change was needed to better communicate to users 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 consistency with the HPC heavy snow forecast and SPC categorical convective outlooks. The previous forecasts were a mix of areal coverage and probability without a intuitive threat scale. The current product starts with a slight (20-39%) threat increasing to moderate (40-69%) and high (70% or greater) of exceeding flash flood guidance.