Verification of NOAA/NWS/SPC 4-Hourly Probabilistic Severe Timing Guidance

The NWS Storm Prediction Center has recently developed probabilistic severe timing guidance, with the goal of providing more specific information regarding the onset, duration, and cessation of severe weather events across the contiguous U.S. (CONUS). This timing guidance combines full-period SPC Day 1 convective outlook probabilities with 4-hr calibrated High-Resolution Ensemble Forecast (HREF) and Short Range Ensemble Forecast (SREF) guidance for severe hazards including hail, tornado, and wind. This timing guidance has been archived since April 2018. To evaluate forecast quality, statistical verification was performed upon a 5-year subset of archived data, comprising 15 April 2018 - 15 April 2023. Performance and reliability diagrams based on the hourly output of the timing guidance accumulated within 4-hr periods and local storm report (LSR) data were constructed. In general, these metrics mimic similar qualities of the stand-alone full-period convective outlook verification, but with variance across the 4-hour periods and a peak in performance in the late evening, coincident with a relative maximum in LSR frequency. In addition, the temporal bias was examined at each gridpoint over the entire CONUS. The timing guidance shows a slight late bias for all hazards, with a gradual increase in bias from west to east.