Weather experts said it's unusual for deadly tornadoes to develop a few weeks apart in the U.S. But what made the two storm systems that barreled through a Missouri city and the South within the last month so rare is that tornadoes took direct aim at populated areas.

Such a pair of weather events is "unusual but not unknown," said tornado researcher Howard B. Bluestein of the University of Oklahoma. "Sometimes you get a weather pattern in which the ingredients for a tornado are there over a wide area and persist for a long time. That's what we're having this year." From the Associated Press.