Rising Hourly Rainfall Intensity
Hourly rainfall intensity—a key factor in flash floods—has increased since 1970 in 136 U.S. locations analyzed by Climate Central.
For every 1°F of warming the air can hold an extra 4% of moisture, increasing the chances of heavier downpours that contribute to the risk of flash floods.
The rapid onset of flash floods limits time to get people out of harm’s way. In the U.S., racial minorities and those living in mobile homes are disproportionately exposed to flood risk, especially in the South and in rural areas.
In the U.S., extreme daily rainfall has become more frequent since the 1980s. Hourly rainfall intensity has also increased since 1970—by 13% on average across 150 U.S. locations analyzed by Climate Central.
And from 1958 to 2016, the heaviest 1% of rainfall events became 55% and 42% wetter in the Northeast and Midwest, respectively—and 9-29% wetter for all other regions.
The latest IPCC reports indicate that precipitation extremes are likely to increase globally, even in regions with decreasing average precipitation.
In the U.S., the Northeast and Midwest are expected to see ~40% increases in the extreme 1% of rainfall events by the end of the century with additional warming.
With high future levels of heat-trapping emissions, U.S. flash floods could also intensify—especially in the Southwest. A recent study suggests that the burdens of an estimated 26% increase in overall U.S. flood risk by 2050 could disproportionately impact Black communities along the Atlantic and Gulf coasts.
Updated: July 2023
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