A Multi-year Evaluation of NSSL-WRF Surrogate Severe Thunderstorm Forecasts

The NSSL-WRF has been run continually since the beginning of 2008 with very few changes to its configuration, allowing for a unique multi-year analysis of this convection-allowing model’s performance in forecasting severe thunderstorms.  In this study, locations at which updraft helicity (UH) values from the NSSL-WRF exceeded a threshold were used as surrogates for reported severe weather events.  The practically perfect smoothing technique was then applied to all surrogate severe reports that were identified during each convective day, resulting in surrogate severe probability forecasts (SSPFs) that can be directly compared to Storm Prediction Center (SPC) convective outlooks.  Using standard verification techniques, the skill of the SSPFs was calculated and analyzed over various time periods, such as annually, seasonally, and by day of the year.  Comparisons are made among SSPFs created using different UH thresholds to highlight seasonal variations in UH performance, as well as with SPC slight risk areas to examine the performance of SSPFs within the context of SPC forecast skill.