<P> The Great Plains, the Midwest and the southern United States - because of contrasting air masses - have frequent severe thunderstorms and tornado outbreaks during spring and summer . In central portions of the US, tornadoes are more common than anywhere else on Earth and touch down most commonly in the spring and summer . The strip of land from north Texas north to Nebraska and east into Southern Michigan is known as Tornado Alley, where many houses have tornado shelters and many towns have tornado sirens . Stretching across Mississippi and Alabama, Dixie Alley has experienced tornadoes and violent thunderstorms, with peak tornado season coming on as early as February and waning by May . Florida also reports many tornadoes but these are rarely very strong . The southern US has a second tornado season during the Fall . In general, the area at greatest risk for tornadoes migrates northward from February to June, peaking in the Gulf States in February and March, the Ohio Valley and lower Midwest in April, southern and central Plains and central Midwest in May, and Northern Plains and upper Midwest (Dakotas, Minnesota and Wisconsin) in June . </P> <P> The Appalachian region and the Midwest experience the worst floods . Widespread severe flooding is rare . Some exceptions include the Great Mississippi Flood of 1927, the Great Flood of 1993, and widespread flooding and mudslides caused by the 1982--83 El Niño event in the western United States . Localized flooding can, however, occur anywhere, and mudslides from heavy rain can cause problems in any mountainous area, particularly the Southwest . The narrow canyons of many mountain areas in the west and severe thunderstorm activity during the monsoon season in summer leads to sometimes devastating flash floods as well, while Nor'easter snowstorms can bring activity to a halt in the New England and Great Lakes regions . </P> <P> In 2013, the US sustains $10 billion annually in damage from floods . </P> <P> The Southwest has the worst droughts; one is thought to have lasted over 500 years and to have decimated the Anasazi people . Large stretches of desert shrub in the west can fuel the spread of wildfires . Although severe drought is rare, it has occasionally caused major problems, such as during the Dust Bowl (1931--1942), which coincided with the Great Depression . Farmland failed throughout the Plains, entire regions were virtually depopulated, and dust storms ravaged the land . More recently, the western US experienced widespread drought from 1999 to 2004 . </P>

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