twister in Flint, Mich., on April 27, 1953.

And the Joplin figure is expected to rise. The National Weather Service said the twister had winds greater than 320 km/h and was an EF5 on the Fujita scale — the highest rating assigned to tornadoes based on the damage they cause.

Gathering storms The huge thunderstorm that spawned the Joplin tornado formed over southeastern Kansas on Sunday afternoon, along the boundary between warm, moist air flowing northwards from the Gulf of Mexico and cold, dry air moving south from Canada. Tornadoes can occur during any month in the U.S. But May, June and July are the most active months, with an average of more than 800. While the numbers this year are only preliminary, more than 1,000 tornadoes were reported upfilled the air in some damaged areas. Unstable buildings also posed threats, as whipping winds threaten to finish them off.

Joplin Mayor Mark Rohr told a media briefing Tuesday morning that the search-and-rescue effort was continuing while the weather remained good, with search dogs and handlers coming "from all over."

Fire Chief Mitch Randles said searchers were still finding survivors and bodies. Seven survivors were found Monday, including one inside the wreckage of a Home Depot outlet.

Asked about looting, a spokesman for the state police said there had been isolated problems, but they had been taken care of. Police are checking everyone leaving the area, he said.

He urged sightseers to stay away. "Yesterday, there was a problem with sightseers coming in," he said. "Every news station is streaming this live