Surrogate Flash Flooding: Probabilistic Excessive Rainfall Predictions from the High-Resolution Ensemble Forecast (HREF) System

Abstract Probabilistic forecasts of excessive rainfall based on the fraction of High-Resolution Ensemble Forecast (HREF) members predicting precipitation above a given threshold are used widely in predicting excessive rainfall; 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 3-yr period, including regional and seasonal variations in skill. We find that there is considerable sensitivity to how excessive rainfall events are defined, especially in regions with large differences in the number of excessive rainfall events between different datasets. When verifying against Stage IV exceedances of flash flood guidance (FFG), both the 0000 and 1200 UTC HREF probabilities of exceeding 6-h FFG thresholds exhibit a higher Brier skill score (BSS) than the operational 0900 UTC day-one excessive rainfall outlook (ERO) in five of eight regions in the contiguous United States (CONUS), while probabilities of exceeding fixed 6- or 12-h precipitation thresholds provide a higher BSS than the ERO in another two regions. There is regional variability in the thresholds providing the highest BSS, with FFG (or 75% of FFG) generally providing the best forecasts in the eastern United States, but fixed thresholds providing the best forecasts in the western United States. Only in the southeastern United States are threshold-based HREF forecasts unable to beat the ERO. The 1200 UTC HREF-based forecasts using regionally optimal thresholds beat the ERO by 25%–30% in terms of BSS. Our results suggest that HREF probabilities of exceeding precipitation thresholds have considerable value for excessive rainfall prediction. Significance Statement Predicting excessive rainfall and flash flooding is a challenging problem. Operational forecasters often use the fraction of high-resolution weather models predicting rainfall above a given threshold as one tool to guide their excessive rainfall outlooks, but it is unclear which threshold they should use or how much skill the resulting probabilities have. Here, we evaluate these probabilities against observed excessive rainfall events and compare them to operational forecasts. If we select the best-performing precipitation threshold in each region, we find that the model-based probabilities are more skillful than operational forecasts in seven of eight regions of the contiguous United States. These results inform forecasters about the best thresholds to use when developing their excessive rainfall outlooks.