Tornadoes are rare. Only a small fraction of thunderstorms produce them, and EF4 and EF5 tornadoes are exceptionally rare, accounting for less than 1 percent of all twisters. Only one EF5 is recorded in the United States in a typical year. In 2011 there were six. Four struck on April 27. Statistically, an event of this magnitude occurs only once or twice a generation. But it can, and will, happen again. And the next one might be bigger.
