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.

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       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.

       Among the flood reports:

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       High water blocked lanes on Interstate 395 before the 14th Street bridge in Arlington County. Columbia Pike was closed in both directions at South Greenbrier Street due to high water in Arlington County. Maine Avenue westbound at Independence Avenue was closed due to high water in Southwest Washington. Vehicles were stuck in high water around Merrifield, Fairfax and Alexandria in Fairfax County. East Braddock Road in Alexandria was closed due to high water, where vehicles were stranded.

       Downed trees, wires and high water reported in parts of D.C. region

       The same humid environment and stalled front that led to the slow-moving downpours in the D.C. area also triggered severe flooding in Richmond. More than three inches fell in an hour in some locations.

       The event was yet another reminder that motorists should not attempt to drive across flooded roads. For many vehicles, it takes just six inches of water for them to stall and a foot to float.

       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.

       The location of the front is shown in the next graphic. An unstable air mass along with deep layer humidity well in excess of average values for mid-September pooled along the front.

       It’s a fact that many flash flood events in and around the D.C.-Baltimore corridor, occurring during summer months, trigger in the vicinity of stagnant fronts. The first thunderstorms that erupted laid down pools of downdraft air (“cool pools”) that helped anchor that further uplift of moist air in place reinforcing rising air along the front.

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       The very weak winds aloft ensured that these cells parked themselves (tall thunderstorm clouds otherwise act like giant “sails” in the winds; strong winds push the cells along).

       Flash floods are multifactorial natural hazards. Intense rain rates and the impervious surface of urban and suburban regions conspire to create the difficult situation. Unlucky timing of lots of vehicles on the roads was the other bad draw of the deck.

       A tough forecast

       Like many flooding events, it proved challenging to predict ahead of time. Capital Weather Gang’s forecast Thursday morning only called for scattered showers and downpours, although the forecast issued on Wednesday afternoon had mentioned the possibility of heavy rain and flooding, noting a “tricky(ish) forecast” for Thursday.

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       Early Thursday afternoon, the Weather Service issued a bulletin cautioning flash flood was possible into the evening. Once the downpours began to develop Thursday midafternoon, we did quickly post to Twitter that we could see flooding develop. Subsequently, the Weather Service issued a flash flood watch for the area, which we immediately shared on social media and in our PM forecast update posted at 3:23 p.m.

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       In hindsight, our Thursday morning forecast should have placed more emphasis on the heavy rain and flood potential given the setup, and we should have called more attention to the possibility sooner on social media.

       Scenes of the flooding from social media

       Columbia Pike in Arlington

       Braddock Road area of Alexandria

       Interstate 395 in Virginia before the 14th Street bridge

       Constitution Avenue in the District

       Metro stations