How and why the D.C. area was deluged by a month’s worth of rain in an hour Monday
By Jason Samenow ,
Ian Livingston and
Jeff Halverson
July 8 at 4:03 PM
A month’s worth of rain deluged the immediate D.C. area early Monday, resulting in one of its most extreme flooding events in years. The record-setting cloudburst unleashed four inches of water in a single hour, way too much for a paved-over, heavily populated urban area to cope with at the height of the morning rush.
The sheets of rain, with nowhere to run off, turned major roads into rivers while streams and creeks shot up 10 feet in less than an hour. The rushing water stranded scores of people in their vehicles, poured into businesses and the Metro system, submerged cars in parking lots, swamped basements and caused some roads to cave in, forming massive sinkholes.
continues at washingtonpost.com
excerpt toward the end of the article:
Some of the highest rainfall totals from across the region in Maryland included North Potomac with 5.55 inches and Gaithersburg with 4.64 inches. In Northern Virginia, top totals included 4.93 inches in Oakton and 4.5 inches in Arlington’s Westover neighborhood. Multiple locations in the District topped 2 inches, including Nationals Park.
Washington’s 3.44 inches of rain, observed at Reagan National Airport, topped its previous July 8 record of 2.16 inches from 1958. It became Washington’s seventh-wettest July day on record (since 1871) and ranked among the top 25 wettest days for the June-to-August summer months. It’s the third time in the past three years Washington has seen a rainfall event ranking among the top 10 wettest for July.
The maximum zone of rain was as narrow as it was intense. Totals in Washington’s western suburbs were considerably lower, with 1.05 inches recorded at Dulles International Airport. At Baltimore-Washington International Marshall Airport, to the northeast, only 0.74 inches fell.
Estimated rainfall from radar combined with rain gauge data, but still probably an underestimate in some areas.
Why it happened
The morning’s very heavy rain stemmed from a conspiracy of circumstances: (1) A stalled front; (2) an area of low pressure that caused moist air to spiral or converge over the D.C. region; (3) an exceptionally humid air mass and (4) slow movement of storm cells.
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The torrential downpours, and ensuing flooding, stem from a classic flash flood-generating setup — a pattern that was very frequent last summer as well. Namely, very high air mass humidity … combined with a stalled front … unstable air … and a disturbance along that front.
Storm environments with these exceptionally high amounts of atmospheric water content are expected to increase from climate change-induced rising temperatures. And it’s plausible Monday’s rainstorm was intensified by the climate warming that has already occurred. |
