How torrential rain flooded parts of the District and Arlington and Fairfax counties Thursday [View all]

[elleng]

'Once again this summer, parts of the District and its western suburbs were besieged by a deluge of rain and dangerous flash flooding. With peak radar-indicated rain totals in the 4 to 5 inches in some spots, it was the “perfect storm” of blinding rain falling on asphalt and concrete surfaces, with inadequate runoff, during rush hour.

Floodwaters engulfed roadways, sent streams surging over their banks and stranded vehicles, mostly in Arlington and Fairfax counties.

The hardest-hit area, where 2 to 4 inches of rain was common, stretched from Arlington and Fairfax to the north and Alexandria and Springfield to the south. Areas of heavy rain surrounded this zone, but amounts were mostly between 1 and 2 inches. As typical with the sporadic nature of summer thunderstorms, some areas missed out on much of the action and received less than a quarter inch.

The National Weather Service issued four flash flood warnings covering the District and its immediate west and southwest suburbs and received more than a dozen reports of high water. . .

How it happened

Thursday’s meteorological setup seemed innocuous enough. The remnants of a cold front stalled out just to the southeast of the District with a warm and humid air mass in place. Jet stream energy from the previous day was moving off to the northeast, over New England. The airflow in the lowest 20,000 feet of the atmosphere had become quite slack.

But in fact, stasis of the airflow was a major contributor to our problem. As the image below shows, a rather concentrated region of thunderstorm cells erupted and remained nearly stationary over the same regions. Gentle but persistent and deep winds from the east-southeast were lifted over the stationary front, triggering the eruption of the storm complex on the northwestern side of the front.'>>>

https://www.washingtonpost.com/weather/2021/09/17/flooding-dc-alexandria-arlington-fairfax/?