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More Midwestern Extreme Storms

The Rocky Mountain Climate Organization and Natural Resources Defense Council in May 2012 released a report, Doubled Trouble: More Midwestern Extreme Storms, which starkly documents how much heavy precipitation has increased in the Midwest and sheds new light on the devastating and costly floods that have hammered the region, especially in recent years.

New RMCO analysis of a half century of precipitation data across the Midwest, defined as Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, and Wisconsin, indicates the region has had an increasing number of extreme storms since 1961. The most intense storms—those of three inches or more of precipitation in a single day—have increased the most, with their annual frequency more than doubling over the past 51 years. The frequencies of all heavy storms, especially the very largest, have particularly spiked this century.

Notable results of the study include:

  • The annual frequency of three-inches-plus storms increased by 103% from 1961 through 2011, while for storms of at least two inches but less than three inches in a day, the trend was an 81% increase; for storms of one to two inches, a 34% increase; and for smaller storms of less than one inch, a statistically insignificant 8% increase.

  • The frequency of extreme storms has increased so much in recent years that the first 12 years of this century included seven of the nine top years (since 1961) for the most extreme storms in the Midwest.

  • A different way to present the same data is in the graphic at the top of this page, which shows the average number of storms per decade, compared to a 1961-1990 baseline.

  • The average return period between two extreme storms at a single location in the Midwest has shrunk from once every 3.8 years in 1961-1970, to once every 2.2 years in the last decade. That recent rate is four to eight times more frequent than landfalling major hurricanes at a typical location along the U.S. coast from Texas to North Carolina.

As Stephen Saunders, president of RMCO and the report's primary author, said: “Global studies already show that human-caused climate change is driving more extreme precipitation, and now we’ve documented how great the increase has been in the Midwest and linked the extreme storms to flooding in the region. A threshold may already have been crossed, so that major floods in the Midwest perhaps now should no longer be considered purely natural disasters but instead mixed natural/unnatural disasters. And if emissions keep going up, the forecast is for more extreme storms in the region.”

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