"Way above average temperatures" are combining with a powerful jet stream to fuel strong thunderstorms, which can turn into tornadoes,
http://www.msnbc.msn.com/id/43164389/ns/weather/?gt1=43001The National Weather Service said the Joplin twister was an EF5, the strongest rating assigned to tornadoes, with winds of more than 200 mph.
"We are still in a search-and-rescue mode," said Mark Rohr, Joplin's city manager. "I want to emphasize that."
Late-night tornado sirens had Joplin's residents ducking for cover again before the storm brushed past without serious problems.