Frequency of Extreme Downpours in the United States, 1948-2011. Extreme downpours have become more frequent across much of the U.S. PennEnvironment, 2012

Global warming is happening now and its effects are being felt in the United States and around the world. Among the expected consequences of global warming is an increase in the heaviest rain and snow storms, fueled by increased evaporation and the ability of a warmer atmosphere to hold more moisture.

An analysis of more than 80 million daily precipitation records from across the contiguous United States reveals that intense rainstorms and snowstorms have already become more frequent and more severe. Extreme downpours are now happening 30 percent more often nationwide than in 1948. In other words, large rain or snowstorms that happened once every 12 months, on average, in the middle of the 20th century now happen every nine months. Moreover, the largest annual storms now produce 10 percent more precipitation, on average.

An increase in extreme downpours has costly ramifications for the United States, with the potential to cause more flooding that jeopardizes property and lives. With scientists predicting even greater increases in extreme precipitation in the years ahead, the United States and the world must take action to reduce pollution that contributes to global warming.

Extreme rainstorms and snowstorms are happening more frequently.

  • Extreme downpours – rainstorms and snowfalls that are among the largest experienced at a particular location – are now happening 30 percent more often on average across the contiguous United States than in 1948. (Our analysis covered the period from 1948 to 2011, which offered the most complete weather data.)
  • New England has experienced the greatest change, with intense rainstorms and snowstorms now  happening 85 percent more often than in 1948. The frequency of intense rain or snowstorms nearly doubled in Vermont and Rhode Island, and more than doubled in New Hampshire.
  • The change has also been pronounced in the Mid-Atlantic, the South, the Midwest and the Mountain West. New York, Pennsylvania and Missouri each experienced an increase in extreme downpour frequency of more than 50 percent.
  • In total, 43 states showed significant increases in the frequency of extreme downpours. Only one state, Oregon, experienced a significant decrease.

When It Rains, It Pours: Global Warming and the Increase in Extreme Precipitation from 1948 to 2011

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