Search for a command to run...
Abstract The High-Resolution Ensemble Forecast (HREF) system is the first operational convection-allowing ensemble in the United States. Its membership consists of five deterministic convection-allowing models, several of which are used in operational forecasting, plus five time-lagged forecasts. HREF forecasts of summertime (JJA) extreme precipitation, defined as exceeding average recurrence interval (ARI) thresholds at 2, 10, and 50 years at 1-, 6-, and 24-h accumulation periods, are evaluated over 3 years (2021–23) with a consistent model configuration. This study evaluates the representation of precipitation extremes by the ensemble, ensemble forecast performance, and the contribution of the individual members to forecasts of extreme precipitation both CONUS-wide as well as over the six different regions. Both observed and predicted ARI exceedances were found to most frequently occur over the western U.S. Forecasts of 6- and 24-h accumulations were found to be reliable for forecast probability < 30% within a 25-km radius of an observed ARI exceedance. Results for forecast probability > 50% were often noisy due to the small sample size of highly predictable extreme events. Forecast skill from both the ensemble and its individual members generally increased with increasing accumulation period and decreased with increasing ARI threshold. Time-lagged members were less likely to contribute to forecasts of extreme precipitation, having lower probability of detection and success ratio than the nonlagged members, though the lag–High-Resolution Rapid Refresh (HRRR) was often an exception. These results provide important, and heretofore lacking, contextual information to forecasters using the HREF and its individual members. Significance Statement The High-Resolution Ensemble Forecast (HREF) system has been running operationally in a constant configuration for 3 years. Despite this, little evaluation has been done on the ensemble and the contribution of its individual members, particularly with regard to the prediction of extremes. Here, we examine forecasts of summertime extreme precipitation from the HREF both CONUS-wide and regionally, including the model climatology of heavy precipitation, ensemble forecasts, and the contribution of individual members to forecasts of extremes. In this way, we provide contextual information that can be used by forecasters using both the HREF and the deterministic models that comprise it in cases of extreme precipitation and by developers working on the next generation of convection-allowing models.