Probabilistic tornado forecasts as an indicator of tornado warning coverage

The Storm Prediction Center (SPC) is the primary agency tasked with forecasting the risk of organized severe convective storms and its associated hazards (wind, hail, tornadoes). The SPC Convective Outlook presents a narrative and graphical synopsis of graduated probabilities where severe weather is expected to occur for the convective day. This publically disseminated product is the first bridge of communication between the SPC and a National Weather Service (NWS) local Weather Forecast Office (WFO) and provides awareness for future warning decision making.

Once a warning is issued, public partners of the NWS (e.g. emergency managers, broadcasters) must act quickly in communicating the perceived hazard and coordinating organizational resources to minimize the loss of life and property. Warnings are short-fuse products that can occur with a minimal lead time. Intermediate products (e.g. Mesoscale Discussion, Watches) provide greater public awareness, but yield a singular level of enhanced risk. Currently, there is little information available before a severe weather event detailing the expected coverage of warnings.

Our study will leverage a three year dataset of storm-based tornado warnings contained within the SPC tornado probability categories. Using Geographic Information Systems (GIS) tools, warning polygons are plotted over the SPC forecasts and their resulting areas are extracted. These extracted areas are then compared to total forecasted area for the convective day. Quantified trends show an increase in warning coverage within higher SPC risk probabilities. The ability to provide a climatological probability of warning frequency and coverage can prove to be an invaluable tool hours before the first warnings are issued. The warning coverage analog defined in this study serves as a proof of concept that, with further refinement, can be used as a decision assistance tool.