Creation and Evaluation of Calibrated SSCRAM-HRRR Severe Hazard Guidance

The Forecasting a Continuum of Environmental Threats (FACETs) research project contains eleven sub projects aimed at developing short-range, probabilistic guidance for many atmospheric hazards. One of our subproject objectives is to develop short-range calibrated probabilistic guidance for tornadoes, large hail, and severe wind utilizing the Statistical Severe Convective Risk Assessment Model (SSCRAM) created by SPC forecasters John Hart and Ariel Cohen (Hart and Cohen 2016). The forecast guidance runs hourly out to 15 hours and is valid at 2-h intervals over the prior 4-h. SSCRAM generates real-time probabilistic hazard guidance for this application from a small number of atmospheric variables forecast by the Rapid Refresh model (RAP) thought to be most closely related to each particular hazard.

The calibrated SSCRAM-HRRR guidance combines time-lagged HRRR numerical model output, using the last four hourly runs of the HRRR, and the SSCRAM probabilistic guidance, which is conditional on the occurrence of a thunderstorm. The SSCRAM guidance for each hazard is taken at each grid point and then multiplied by the 4 hourly probability of simulated reflectivity being greater than or equal to 40 dBZ from the time-lagged HRRR, which is used as a proxy for the probability of a thunderstorm. This creates an “unconditional” probability of each severe hazard valid over a 4-h period. The guidance was utilized and evaluated in the 2017 Hazardous Weather Testbed Spring Forecasting Experiment. Subjective evaluation and statistical verification shows that the product can provide useful guidance to severe weather forecasters.