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Quick Answer. Tornadoes can last from a few seconds to over an hour, although most last less than 10 minutes. They form from mesocyclones, which are areas of organized rotation within supercell thunderstorms. Scientists still don't know why some mesocyclones become tornadoes and some do not.
NWSâs preliminary estimate is that there have been 1,691 tornadoes in 2011. The previous yearly record number of tornadoes was set in 2004 with 1,817. The overall yearly average number of tornadoes for the past decade is 1,274. The preliminary estimated number of tornado fatalities for 2011 is 550. NWS records indicate that there were 364 tornado fatalities before the Joplin event.