Following an investigation of a "high" EF-3 tornado that struck Tuscaloosa, Ala., in 2011, a technical-assessment team for the American Society of Civil Engineers recommended that Tuscaloosa and other jurisdictions in "Dixie Alley" prioritize construction of community shelters for trailer parks and elementary schools.

"In one instance, we encountered a residential structure reduced to rubble except for the above-grade safe room it contained," says John van de Lindt, an engineering professor with Colorado State University, Fort Collins, who participated in the investigation.

In 2011, Oklahoma state school officials credited a pair of safe rooms for protecting children from an EF-3 tornado that struck a school in Tushka. One structure was below grade, and the other was above grade and adjacent to the school.

Meteorological data indicate EF-4 and EF-5 tornadoes have not increased in frequency, though the Moore and Joplin events demonstrate their potential impact on populous areas, says Joplin building official Steve Cope. "It's not just isolated farmhouses out here any more," he says. "You've also got good-size communities standing in the track of these storms."