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How and why the D.C. area was deluged by a month’s worth of rain in an hour Monday

The D.C. region was hammered by heavy rain and flash floods on July 8. (Video: The Washington Post, Photo: Peter Jamison/The Washington Post)
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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.

Floodwater poured from the ceiling on the tracks at the Virginia Square-GMU metro station in Arlington, Va., on July 8. (Video: Niina Farah via Storyful)

Montgomery County, northern Fairfax County and Arlington endured some of the most extreme rainfall.

At Reagan National Airport, Washington’s official weather observing location, 3.44 inches fell, with 3.3 inches coming in the 9 a.m. hour alone. The odds of rain this intense in any given year are less than 1 percent.

How the event unfolded and how much rain fell area-wide

A complex of slow-moving thunderstorms formed in northern Maryland overnight and intensified around Frederick around dawn, where more than four inches of flooding rain poured down. Over the coming five hours, the supercharged storm complex would emit more than 4,000 lightning flashes on its southeastward trek through the Beltway and Southern Maryland.

Flash flood warnings were first issued in the Washington region between 7 and 8 a.m. as the storm complex oozed southeast along the Interstate 270 corridor. Excessive rain quickly overwhelmed creeks and streams, forcing the Montgomery County Fire Department to respond to dozens of calls for swift water rescues.

The cluster of storms arrived near Great Falls around 8:30 a.m., where the water level at Difficult Run, which feeds the Potomac River, climbed four feet in 30 minutes. Minutes later, it reached Northwest Washington and Arlington, causing Rock Creek to surge and Arlington’s Four Mile Run to rise over 11 feet in an hour (between 8:45 and 9:45 a.m.)

As the storm complex slugged through Alexandria and downtown Washington between 9 and 10 a.m., Cameron Run in Alexandria rose nearly 12 feet. It was in this hour that the National Weather Service declared its first-ever Flash Flood Emergency for the District (such alerts began in 2011), reserved for the most serious events which are “life-threatening” and “particularly dangerous” situations.

The maximum observed rainfall rates associated with the storms were incredible, reaching 4 to 6 inches per hour.

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.

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.

The key elements described above are shown in the figure above. A frontal boundary was draped through the immediate Washington region (having sagged slowly south out of Pennsylvania overnight). This front, in and of itself, caused low level, moist air to rise.

But an additional impetus — a small zone of low pressure, called a mesolow, developed just northwest of the District along the front. This low began pulling in low-level, moisture-laden winds, as shown by the dark red, spiral arrows.

A cluster of convective cells (with lightning and thunder) developed in this zone of converging air and moved slowly toward the southeast. There was enough unstable air present, even at sunrise, to trigger these storms.

The air mass was exceptionally humid, approaching record levels for July over Washington.

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.