As the calendar advances through March, many Chicagoans grow increasingly anxious for the return of mild spring weather. That’s especially the case when wintry temperatures refuse to relax their grip on the area and the forecasts often contain a mention of snow, as has been the situation this March. It’s easy to overlook the climatological fact that Illinois’ severe weather season begins in March. Indeed, the most destructive and most deadly tornado in U.S. history, the Tri-State Tornado of March 18, 1925, was primarily an Illinois event. The tornado claimed 695 lives and injured 2,000, most of them in southern Illinois. The present 7-day forecast cycle anticipates a continuation of subnormal temperatures. And, despite our impatience for warm weather, there is a significant benefit: The chill greatly diminishes the threat of severe weather.