Repeat deadly storms "unusual but not unknown"

By RANDOLPH E. SCHMID | May 24, 2011 | 4:27 AM EDT

James Miller tosses aside a door as he searches through what remains of his sister's house Monday, May 23, 2011, in Joplin , Mo. A large tornado moved through much of the city Sunday, damaging a hospital and hundreds of homes and businesses. (AP Photo/Jeff Roberson)

WASHINGTON (AP) — Experts say it's unusual for deadly tornadoes to develop weeks apart in the U.S. But what makes the tornadoes that barreled through a Missouri city and the South in the last month so rare is that populated areas took direct hits.

The tornado in Joplin, Mo., on Sunday killed at least 116 people in the deadliest single tornado in almost six decades. Twisters that swept through the South last month killed more than 300.

University of Oklahoma researcher Howard Bluestein says such a pair of weather events is "unusual but not unknown." He says tornado-appropriate weather sometimes lingers over a wide area. That's what's happening this year.

Experts say urban sprawl has increased the odds that tornadoes will affect more people. Forecasters have little time to determine a tornado's path.