<P> Most tornadoes in the United States occur east of the Rocky Mountains . The Great Plains, the Midwest, the Mississippi Valley and the southern United States are all areas that are vulnerable to tornadoes . They are relatively rare west of the Rockies and are also less frequent in the northeastern states . Tornado Alley is a colloquial term for an area particularly prone to tornadoes . There is no officially defined' Tornado Alley' - at its broadest this area stretches from North Texas to Canada with its core centered on Oklahoma, Kansas and northern Texas . Another highly significant region - colloquially known as Dixie Alley - is the southern United States and particularly the northern and central parts of Alabama and Mississippi . Florida is one of the most tornado prone states . However, Florida tornadoes only rarely approach the strength of those that occur elsewhere . </P> <P> Although favorable conditions for tornadoes in the United States can occur at any time, they are most common in spring and least common in winter . Because spring is a transitional period for the climate, there are more chances of cooler air meeting with warmer air, resulting in more thunderstorms . Tornadoes can also be spawned by landfalling tropical cyclones, which usually occur in late summer and autumn . In the United States, thunderstorms capable of producing tornadoes usually form when the temperature is at its highest, typically from 4: 00 p.m. to 7: 00 p.m. </P> <P> Although the period in which most tornadoes strike ("tornado season") is March through June, tornadoes - including violent tornadoes and major tornado outbreaks - have been documented in the United States during every month of the year . Two examples of this are when a series of tornadoes hit the state of Indiana on November 22, 1992, and injured at least nine people . Another notable non-season tornado was where a tornado struck the area of McLean County, Illinois . Even though the tornado was during a winter month, it blew 20 railroad cars off their tracks, and hauled a camper over 100 yards (91 m). </P> <P> During the winter months of the year, tornadoes have been known to hit the Southern United States and Southeastern United States the most, but have hit other areas as well . One notable recent example of a winter tornado outbreak was the 2008 Super Tuesday tornado outbreak on February 5 and February 6, 2008 . 84 tornadoes occurred over the course of the outbreak . The storm system produced several destructive tornadoes in heavily populated areas, most notably in the Memphis metropolitan area, in Jackson, Tennessee, and the northeastern end of the Nashville metropolitan area . At least 57 people were killed across four states and 18 counties, with hundreds of others injured . The outbreak was the deadliest of the modern NEXRAD doppler radar era, until the 2011 Super Outbreak killed over 348 people (324 of which were tornado - related). It was the deadliest single outbreak since the May 31, 1985 outbreak, which killed 76 across Ohio and Pennsylvania, as well claiming 12 victims in Ontario, Canada . It was also the deadliest outbreak in both Tennessee and Kentucky since the 1974 Super Outbreak . </P>

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