Around 21% of the U.S. can expect '100-year flood' events every 25 years

For around 21% of the U.S., severe 1-in-100-year flood events can now be expected to occur every 25 years, according to an updated precipitation model from the First Street Foundation, which projected that 20 counties across the country can expect to see a 100-year flood event every 8-10 years in the most extreme cases.

The projections are based on an updated precipitation model that includes climate-driven changes in heavy rainfall events, which had previously been unaccounted for in the National Oceanic and Atmospheric Administration’s (NOAA) Atlas 14 precipitation frequency data tool.

“This work highlights the degree to which the changing climate has affected our understanding of the likelihood of extreme precipitation events,” Dr. Jeremy Porter, head of climate implications research at the First Street Foundation, said in a release. “Over the last few years, we have seen a remarkable uptick in flooding from heavy rainfall, which, unfortunately, is becoming our new normal for most of the U.S. population.”

First Street’s study projects that 51% of the U.S. population is living in an area that is now twice as likely to experience a severe flood compared with Atlas 14 projections. NOAA projected that around 44% of the U.S. was at risk of spring flooding.

The Northeast, Ohio River Valley, and the Texas and Louisiana Gulf Coasts saw the biggest correction in the expectation of heavy rainfalls. While these regions have historically invested heavily in flood protections, those projects were designed to standards based on Atlas 14 forecasts and are likely to fail, First Street reported.

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