The United States, especially that portion east of the Rocky Mountains, is the tornado capitol of the world, as this month has clearly demonstrated. Having served up three times the average number of tornadoes for April and closing in on the all time monthly record of 543, set in May of 2003, the severe weather outbreaks of April, 2011, have produced almost 500 twisters. Yesterday's swath of deadly storms, which stretched from Mississippi to Georgia, was second only to the super-outbreak of April, 1974, and killed at least 170 while leaving a path of destruction almost 300 miles long; more tornadoes are forecast to plague the mid-Atlantic region today.
Lying between a steady source of cool air from Canada and the warm waters of the Gulf of Mexico, the central and southeastern U.S. is often caught in a clash zone as Pacific storms push across the Rockies and intensify over the Great Plains. This scenario, produced by geography and our regional climate, has been in place for at least 70 million years, interrupted only by periods of glaciation, the last of which ended 10,000 years ago.
The weather records by which we compare modern storm systems cover just 200 years, a snapshot in geologic time, and may or may not be significant from the perspective of natural history; needless to say, the destructive power of these recent outbreaks has been of major significance to human communities. Whether episodes of severe weather will increase in concert with global warming is hard to predict but, based on the events of recent decades, it appears that a trend is developing.