Tornadoes in the United States

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Tornadoes are more common in the United States than in any other country or state.^[1][2] The United States receives more than 1,200 tornadoes annually—four times the amount seen in Europe.^[3][4] Violent tornadoes—those rated EF4 or EF5 on the Enhanced Fujita Scale—occur more often in the United States than in any other country.^[3]

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 northern 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.

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.^[5][6] 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.^[6][7]

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 and day of the year.^[8] 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.^[8] 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).^[8]

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.^[8] One notable recent example of a winter tornado outbreak was the 2008 Super Tuesday tornado outbreak on February 5 and February 6, 2008. 87 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.^[9] 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.^[10]

Usually, tornadoes hit specific areas of the United States in specific seasons. During the winter months, tornadoes are usually spotted in the Southern area of the country, as well as states near the Gulf of Mexico.^[11] This is due to cold air moving southward reaching its southern limit of expansion, and stopping over the Gulf Coast.^[11] As spring comes, hot air progressively moves back into the Gulf Coast. This pushes the mass of colder air forward out of the Gulf States and into the Southeastern states, where tornado frequency is highest in April.^[11]

As spring passes and summer begins, the mass of warm moist air moves northwest into the Great Plains and Midwestern states.^[11] During the months of May and June, tornado activity is as its peak in the southern Great Plains. The air mass then moves northward into the Northern Great Plains and the Great Lakes area, causing a tornado activity peak in these areas during the summer months.^[11] During the late summer and early fall months, tornado activity in the United States tapers off.^[11] This is due to the relatively small difference between the temperature at the boundary of the hot air mass and the cool air mass at that time and an extension of the Bermuda High sitting over parts of the United States.^[11] Though there may be some thunderstorms, they don't often become severe enough to spawn tornadoes.^[11]

Tornadoes may be formed out of season, especially during the months of hurricane season in the Gulf Coast states and Southeastern states. Because these areas are prone to hurricanes, they may be struck with tornadoes that are spawned from hurricanes.^[12] Tornadoes are most likely to form in the right-front quadrant of the hurricane, but can also form in rain bands associated with the storm.^[12] This is caused by the large amount of vertical wind shear to the right of the storm.^[13] Tornadoes are also spawned from U.S. hurricanes due to the moistness of the air at the landfall of the storm, which makes conditions favorable for a supercell storm to develop within the hurricane.^[14]