WASHINGTON -- 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.

The tornado that hit Joplin, Mo., on Sunday killed more than 100 people and marked the nation's deadliest single tornado in almost six decades. The series of twisters that swept through the South late last month killed more than 300 people. Both disasters leveled entire communities.

Such a pair of weather events is "unusual but not unknown," said tornado researcher Howard 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."

And the threat is continuing, he said, noting that more storms are predicted over the next few days.

Other than the death toll, there was nothing too unusual about the Joplin storm, he said. The conditions were right and thunderstorms were forecast.

"This is a situation where the tornado went right through a town. If it had been 10 miles away, far fewer people would have been affected," Bluestein said.

Urban sprawl has increased the odds that tornadoes will affect more people, said Joshua Wurman, president of the Center for Severe Weather Research in Boulder, Colo. He likened the situation to barrier islands, where more and more homes are being built in areas prone to hurricanes.

Forecasters can't tell very far in advance where the path of destruction is going to be, said Greg Carbin, warning coordination meteorologist for the Storm Prediction Center in Norman, Okla. A lot of tornadoes hit open spaces, so "when you move to major population centers, the death toll can climb."