An old folk saying states “As the days grow longer, the storms grow stronger.” It’s not surprising that there is a physical connection to the old saying. Longer days mean more hours of sunshine. This in turn, provides more energy to the atmosphere which would fuel the strong storms. Couple that increased energy with a strong contrast in air masses and you have all the typical components for a severe weather outbreak. It is not surprising then to find that the long-term average has the peak tornado month as May, when days are approaching their greatest length, but strong cold fronts are still possible. Just remember, severe weather can occur anytime! The December 2016 tornado outbreak is a very recent reminder of that fact.
Mary Knapp, Weather Data Library