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In the last few years, U.S. residents have seen our fair share of severe weather and climate events—Superstorm Sandy in the Mid-Atlantic and Northeast, severe drought and wildfires in California and much of the West, flash flooding in Colorado, tornadoes in the Great Plains, Midwest and Southeast, and recent snow storms along the East Coast, to name a few. It may seem like we’re experiencing more severe and intense weather events today than a few decades ago, but what does the science say about the long-term trends?
- Overall, extreme precipitation has increased in frequency and intensity. Studies of extreme precipitation show there is an average upward trend in frequency and intensity at a national scale. Trends at a regional level are more varied. Increases have been recorded in the central United States to the North Atlantic. For example, the central United States has had a 40 percent increase in the number of single day and multiday extreme rain events, whereas there are no significant extreme precipitation trends in the Western United States.
- Tornadoes, hailstorms and thunderstorms do not show significant trends. There is no trend in the occurrence of F1 or stronger tornadoes on the Fujita scale since 1954, and there is considerable variability in the incidence of hailstorms. A change in methods used to collect data has led to little confidence in the accuracy of trends; human observations were replaced by automated stations in the 1990s, influencing the comparability of past and future observations.
- Hurricanes and typhoons are still mysterious phenomena across long time scales. Scientists have not been able to quantify the natural variability of these events and their physical linkages to climate forces remain uncertain. Changes in technology and methodology have contributed to differences in the data collected. In the 1940s tropical cyclones were studied by aircraft, but satellites came along in the 1960s. In 1987, the cessation of aircraft monitoring of western North Pacific typhoons created a void of data and knowledge.
- Extreme snow storms have increased in number in recent decades. There have been twice as many extreme snowstorms in the past half century as there were in the preceding one, but since 1900, no significant changes in their U.S. distribution were detected.
- Heat waves have increased in frequency; so have the number of record hot days and nights. Studies have shown an increase in record-breaking heat waves in many parts of the world. Globally, the number of warm nights increased by about 25 and the number of cold nights decreased by about 20 over the period from1951-2003. The number of record hot days doubles the number of record cold days in the United States.
- Floods show both increasing and decreasing trends. Flood magnitudes have been decreasing in the Southwest and increasing in the northern half of the eastern prairies and parts of the Midwest. The decrease in the Southwest could be attributed to drying and diminishing snowpack.
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