It is a threat all over the United States and occurs nearly every day. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts too fast, or when dams or levees break. The most destructive and deadly tornadoes come from rotating thunderstorms called supercells and can have winds estimated at more than 200 mph.įlooding is an overflowing of water onto land that is normally dry. Hailstones can grow to several inches in diameter and may fall at speeds greater than 100 mph.Ī tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they can freeze into balls of ice. Most thunderstorm winds that cause damage at the ground are a result of outflow generated by a thunderstorm downdraft. Strong winds are often called “straight-line” winds to differentiate the damage they cause from tornado damage.

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