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.