Extreme weather includes unexpected, unusual, unpredictable, severe or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Often, extreme events are based on a location’s recorded weather history and defined as lying in the most unusual ten percent. In recent years some extreme weather events have been attributed to human-induced global warming, with studies indicating an increasing threat from extreme weather in the future.
Total winter precipitation in the United States had increased by 10 percent since 1900 and that "extreme precipitation events" -- rainstorms that dumped more than two inches of water in twenty-four hours and blizzards -- had increased by 20 percent.
Global warming models indicate that rising global temperatures are likely to affect many atmospheric parameters including precipitation and wind velocity, and raise the incidence of extreme weather events, including storms and heavy rainfall, cyclones and drought. It may or may not be just coincidence that the Munich Reinsurance Company recorded more than 700 'large loss events' in 1998, compared with between 530 and 600 during previous recent years. The most frequent natural catastrophes were windstorms (240) and floods (170), which accounted for 85 per cent of the total economic losses (Munich Re 1998).
Engineers designing storm sewers, bridges, and culverts used to plan for what they called the "hundred-year storm." That is, they built to withstand the worst flooding or wind that history led them to expect in the course of a century. That history no longer applies. "There isn't really a hundred-year event anymore... we seem to be getting these storms of the century every couple of years." The flood of the Red River over Grand Forks, North Dakota in the spring of 1997 was referred to by some as "a 500-year flood" -- meaning, that prediction of the old kind is finished; that these are not acts of God.