"Surrogate Flash Flooding": Probabilistic Excessive Rainfall Predictions from the High Resolution Ensemble Forecast Href System

The High Resolution Ensemble Forecast (HREF), an ensemble of opportunity constructed from operational convection-allowing model (CAM) guidance, is the current operational ensemble system for short-range convection-allowing forecast problems in the US. Probabilistic forecasts of excessive rainfall based on the fraction of HREF members predicting precipitation above a given threshold within a spatial neighborhood are used widely in operational forecasting; however, there is not yet a published study evaluating the skill of these forecasts. In this study, we document the performance of these forecasts over a three-year period, including regional and seasonal variations in skill, with a focus on highlighting the best performing precipitation thresholds and accumulation intervals.

We find that, in seven of eight CONUS sub-regions, 1200 UTC HREF forecast probabilities of exceeding Flash Flood Guidance (FFG), Average Recurrence Interval (ARI), or fixed precipitation thresholds provide an excessive rainfall forecast with higher Brier Skill Score (BSS) than the operational WPC 0900 UTC excessive rainfall outlook (ERO). There is considerable regional variability in the thresholds providing the highest BSS, with FFG (or 75% of FFG) providing the best forecasts in three of eight CONUS sub-regions. In the western US, fixed precipitation thresholds of 50.8 mm 6 h-1 or 50.8 mm 12 h-1 lead to the most skillful forecasts. Only in the southeastern US are threshold-based HREF forecasts unable to beat the 0900 UTC excessive rainfall outlook. Probabilities from the 0000 UTC HREF are able to beat the WPC 0900 UTC ERO in about half of the CONUS sub-regions.

Our results suggest that HREF probabilities of exceeding precipitation thresholds have considerable value for excessive rainfall prediction. These “surrogate flash flooding” predictions can also serve as a natural baseline forecast for more sophisticated forecast approaches for excessive rainfall, analogous to approaches for predicting severe convective hazards.