Ensemble Forecasts and Verification of the May 2015 Multi-Hazard Severe Weather Event in Oklahoma

Dual threat severe weather events in which both tornadoes and flash floods affect the same area within a short time frame pose a complex problem given that the life-saving actions for these two events are contradictory. One such event is the 6-7 May 2015 tornado and flash flood event over Oklahoma. This study explores the capability of a rapidly-updating 3-km horizontal grid spacing convective-scale ensemble data assimilation and prediction system developed as part of the Warn-on-Forecast initiative to forecast features of this dual threat severe weather event. Results indicate that the 0-1 h probabilistic forecasts of reflectivity verify reasonably well with the observations. However, beyond the one hour forecast period, the forecast accuracy is degraded, including biases in storm motion as well as spurious cell generation. The ensemble probability matched mean quantitative precipitation forecasts capture the placement of most intense areas of precipitation very well, but underestimate the amount of accumulated precipitation. These quantitative precipitation forecasts are found to outperform the deterministic quantitative precipitations forecasts of the operational High-Resolution Rapid Refresh model for this case as well. Additional ensemble forecast experiments achieved by simple downscaling to 1-km grid spacing from the 3-km ensemble introduce more spurious cells than found in the original results and suggest that data assimilation at 1-km grid spacing may be necessary.