NSSL scientists have been collaborating with the Storm Prediction Center (SPC) to build statistical models of severe weather occurrence for any day of the year in the United States. The models tell us when and where severe weather happens anywhere in the country and how consistent it is from year to year. They are based on reports of severe weather as far back as 1921.

The models provide information for many users. For forecasters, they give an estimate of the long-term probability of an event as a starting point for making a forecast for a particular day. For emergency managers and the risk industry, they show the overall threat of severe weather at any place in the country. And, because the models provide an estimate of variability, they can be used to evaluate the likelihood of observed events. In addition, they form a baseline for possibly detecting changes in severe weather frequency associated with climate change.