Development and Evaluation of Probabilistic Severe Weather Guidance with the NCAR Convection-Allowing Ensemble

Since Spring 2015, an experimental, real-time, full-CONUS, convection-allowing (3-km horizontal grid spacing) ensemble forecast system has been running at NCAR. Daily 10-member ensemble forecasts are initialized at 0000 UTC and integrated through 48-hours. The ensemble forecasts provide a unique dataset to explore the generation of probabilistic severe weather guidance using explicit forecasts of convection. This presentation will highlight efforts to calibrate and refine probabilistic severe weather guidance using storm surrogates related to severe weather, such as updraft helicity (UH) to detect simulated mesocyclones, specifically for the Day 1 (forecast hours 12 through 36) and Day 2 (forecast hour 36 through 48) periods. In addition to using UH to generate ensemble surrogate severe probabilistic forecasts (E-SSPFs) for all severe weather threats, E-SSPFs were developed for specific severe weather hazards, such as using explicit forecasts of low-level rotation to create probabilistic tornado forecasts and using hail size diagnostics to produce probabilistic hail forecasts. Verification of the forecasts using storm reports during 2015 and 2016 will be presented and forecast performance as a function of season and region will be discussed. Finally, E-SSPF skill will be compared to SPC forecast skill to provide a sense of model skill in the context of human forecast performance.