The Science Behind Washington's Scary 'Flash Flood Emergency'

A storm caused extreme flooding in parts of Washington, D.C., including in East Potomac Park where you could dangle your feet in the water while sitting on a park bench, on July 8, 2019. (Image credit: Jim Watson/AFP/Getty Images)

"TURN AROUND DON'T DROWN!"

That was the all-caps message the D.C./Baltimore National Weather Service (NWS) issued today (July 8) as surging water engulfed roads and stranded drivers — requiring multiple water rescues. The severe flooding prompted the office to issue a somewhat unusual "flash flood emergency" declaration in Washington D.C. and its Virginia suburbs. That's one step up from the somewhat more common "flash flood warning."

Even the White House basement reportedly flooded.

A slow-moving, hard-pounding cluster of storms was responsible for the high waters, according to Cody Ledbetter, a NWS forecaster in the Virginia-based D.C./Baltimore office.

Those storms were part of a hot, wet air mass — called a boundary layer — that formed near the northern tip of the state near the West Virginia border and flowed south and east toward the capital.

"We had lots of reports of 3 to 5 inches [of rainfall] across the region, from Frederick city [in western Maryland] to D.C., and even some reports a bit higher than that," Ledbetter told Live Science.

The critical factor for turning all that rain into a flood was the speed with which it fell, he said.

"It fell in the span of two hours or less. There's no way the ground could keep up with that. It was coming down way too fast for the ground to be able to soak any of it up," he said. "So it flows pretty much immediately into streams, rivers. That causes them to rise pretty quickly and overtake roadways."

The good news, he said, is that flash floods should abate within hours as the rain stops and water flows downriver. In the meantime, the NWS is urging residents to avoid dangerous flooded roads, and find alternate routes to avoid stranding or drowning.

Originally published on Live Science.

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Rafi joined Live Science in 2017. He has a bachelor's degree in journalism from Northwestern University’s Medill School of journalism. You can find his past science reporting at Inverse, Business Insider and Popular Science, and his past photojournalism on the Flash90 wire service and in the pages of The Courier Post of southern New Jersey.