Immense rains are causing more flash flooding, and experts say it’s getting worse

by Tim Craig and Angela Fritz June 24, 2018 at 7:05 PM

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Torrential rain events across the United States are becoming more frequent and more intense, leading to record rainfall, rare extreme flooding and perilous infrastructure failures.

Experts say the immense rains — some spawned by tropical ocean waters, others by once-routine thunderstorms — are the product of long-rising air temperatures and an increase in the sheer size of the storms. Because warmer air can hold more water, large storms are dropping far more rain at a faster clip.

Such rains in recent weeks have deluged the Great Lakes region, the Deep South and the suburbs of major cities along the Atlantic coast. Philadelphia, Charlottesville, and Ocean City, Ellicott City and Frederick in Maryland all have experienced major flooding since mid-May. Several locations in Maryland had their wettest May on record, including Baltimore, which tallied more than eight inches, most of which fell in the second half of the month.

“Things are definitely getting more extreme,” said Andreas ­Prein, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo. “You just have to look at the records. All areas of the continental U.S. have seen increases in peak rainfall rates in the past 50 years. . . . And there is a chance that we are underestimating the risk, actually.”

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Several atmospheric researchers said in interviews that they agree with that perception. They say it is getting worse.

Since 1880, global temperature has risen just more than 0.13 degrees per decade, for a total of 1.8 degrees Fahrenheit (1 degree Celsius). The amount of water air can hold is based on temperature — put very simply, the warmer the air is, the more water it can hold.

Theoretically, experts say, an additional 1.8 degrees would amount to about 7 percent more water in the air, resulting in a similar increase in extreme rainfall. But what Prein and other researchers have found is much higher across a vast portion of the United States.
[Seems to me it could result in a larger percent increase, depending on the location. The atmosphere is a container which can hold a certain amount of moisture at a given temperature. Once the maximum is reached, I would think the same percent change would make for relatively much more precipitation. I will try to get an expert opinion on my hypothesis.]

According to the 2014 National Climate Assessment, the eastern half of the continental United States has seen the most dramatic change in extreme rainfall. The amount of rain during the most extreme storms in the Northeast has risen 71 percent since 1958; in the Midwest, heavy rain has ­increased 37 percent; in the Southeast, it’s up 27 percent.

And the area covered by each storm also is getting larger, Prein said, another major factor in the increased precipitation. Prein’s new research suggests thunderstorms will become 80 to 90 percent larger by the end of the century.

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