<P> Tornadoes are more common in United States than in any other country . The United States receives more than 1,200 tornadoes annually--four times the amount seen in Europe . Violent tornadoes--those rated EF4 or EF5 on the Enhanced Fujita Scale--occur more often in the United States than in any other country . </P> <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>

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