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Fri Sep 11, 2020, 08:34 PM

How Thursday's flood unfolded in the Washington region and why it was so hard to forecast

'During the afternoon Thursday, a torrent washed over the heart of the D.C. area, unloading up to a half foot of rain in just a few hours. The deluge caused creeks and streams to rise up to eight feet in a single hour and overwhelmed drainage systems, bringing widespread flooding that inundated roads and stranded dozens of motorists.

The very narrow zone of heavy rainfall resulted from an extremely moist air mass, converging air and a weak disturbance over the area. These ingredients indicated the potential for heavy rainfall somewhere in the region, but predictive models did not show it occurring where and when it did.

The National Weather Service received more than 40 reports of flooding, concentrated in the zone from Alexandria through southern Montgomery County and extending east through the District and the northern and western parts of Prince George’s County. In this zone, two to six inches of rain were widespread.

The heaviest rainfall totals, exceeding four inches, focused in the area from Northeast D.C. through College Park to around Adelphi in Prince George’s County. Hyattsville received 6.35 inches, the highest amount recorded.

Reagan National Airport received 2.88 inches from this event, a record for the date and the heaviest single-day rainfall in 2020. Capital Weather Gang’s Ian Livingston noted it was the fourth time this year that at least two inches of rain fell, tied for the sixth most on record.

Dulles International Airport and Baltimore-Washington International Marshall Airport only received 0.11 and 0.47 inches from the event, showing the localized nature of the heaviest rainfall, which fell in a narrow corridor.

How it happened and the forecasting challenge
The National Weather Service had placed a long section of the Interstate 95 corridor from southeastern Virginia to Connecticut under a slight risk for flash flooding through the day and evening. Our area was also under a flash flood watch.

Our experience over the years is that somewhere within these broad threat zones, one relatively small area typically experiences the right combination of factors for a flash flood. The problem is a priori identifying that one small zone, which has proved notoriously difficult to do, even with the most sophisticated, high-resolution models.'>>>


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