Impacts of Distance from the Nearest Radar, Time of Day, Resident Population, and Season on Severe Warning Performance. Part II: Regional Analysis

Severe weather warnings alert the public about imminent threats to life and property. While there has been research into how tornado warning verification varies by time of year and time of day, a similar study has not been conducted for severe thunderstorm warnings, which make up the majority of convective warnings issued across the continental United States. By exploring severe thunderstorm warning verification, we aim to identify performance issues so National Weather Service meteorologists can focus training and future research to improve warning services. Our study shows how severe thunderstorm warning performance varies by time (hour, month, season), population impacted, and various WSR-88D factors (distance from the nearest radar, primary or secondary Weather Forecast Office radar). We further stratified the role that non-meteorological factors may play in warning performance due to spotter availability, decreased sampling as distance from the nearest radar increases, and differences in performance between geographic regions of the continental United States. We evaluated 9 years of warnings and events (2010-2018), dating back to the last change in severe thunderstorm warning criteria, the switch from ¾ inch (19.1 mm) to 1 inch (25.4 mm) hail.

Preliminary results focusing on the north central United States suggest detection of wind events lags behind hail events, regardless of time of day, and warning performance is best during the afternoon and evening, regardless of event type. The probability of detection (POD) was higher for hail than wind events, independent of distance from the radar - except for storms within 40 km of the nearest radar where performance was similar. There was a notable increase in false alarm rate (FAR) overnight, with values greater than 0.5 between 0500 UTC and 1000 UTC while POD was less than 0.8 for several hours in that time range. Lower FAR, less than 0.4, and higher POD, greater than 0.85, were found during the late afternoon and evening (2100 UTC - 0200 UTC). By expanding this study to include the entire continental United States, we aim to identify both general performance concerns and difficulties specific to certain regions.