<Tr> <Td_colspan="14"> Source: NOAA (US) </Td> </Tr> <P> Because of contrasting air masses, the Great Plains, the Midwest, and the southern United States have frequent severe thunderstorms and tornado outbreaks during both the spring and the summer . In central portions of the U.S., tornadoes are more common than anywhere else in the world . They usually can touch down during the spring and the 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 where many towns have tornado sirens . Stretching across Mississippi and Alabama, Dixie Alley has experienced both 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 rarely are very strong . The southern US has a second tornado season during the autumn . Generally, 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> Both 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 . Mudslides from heavy rain can even cause problems in any mountainous areas, particularly in 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 . On the other hand, Nor'easter snowstorms can bring activity to a halt in both the New England and Great Lakes regions . </P> <P> In 2013, the US sustains $10 billion annually in damage from floods . </P>

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