Northern D.C. area hit by flash floods. Here’s why it happened.

Several parts of the northern half of the D.C. area were overwhelmed by flash flooding late Saturday afternoon and evening because of intense, slow-moving thunderstorms. Silver Spring, Maryland, and Great Falls, Virginia, were the hardest hit after 5 to 5.5 inches fell in less than two hours. Flooding also extended into northern sections of D.C. and Prince George’s County.

The deluge caused waterways to overflow their banks, transformed roads into rivers and stranded vehicles in high water. The torrents were fueled by exceptional humidity levels that have hovered at near-record levels for weeks.

As vast volumes of water poured into area streams, they rose with remarkable haste. A sensor along Sligo Creek, which winds through Silver Spring and Takoma Park, reported a 10-foot rise in water in just 30 minutes. The creek crested at its highest level in at least 20 years in Takoma Park.

Area watersheds such as Sligo Creek were under significant stress from days of heavy rain over the past week. With soils saturated, the tipping point to the flash flood stage was rapidly reached.

Because of the abrupt water rise and ongoing downpour, the National Weather Service declared a flash flood emergency — its most dire flood alert — for Silver Spring and its surroundings.

Dozens of water rescues were carried out in southern Montgomery County; many around Silver Spring and spanning from Potomac to Takoma Park, according to Pete Piringer, chief spokesperson for the county’s fire and rescue service.

The flooding in Great Falls closed portions of Georgetown Pike and prompted rescues of multiple people stranded in high water on Old Dominion Road.

Flooding was also reported in northern Prince George’s County around Hyattsville and College Park.

Peak rainfall rates reached at least 2 to 4 inches per hour. College Park registered a record hourly amount of 2.71 inches between 5 and 6 p.m.

A large area from roughly Great Falls to Greenbelt, Maryland, received at least two inches of rain.

How the flooding unfolded

Through much of the day, the region’s main flash flood concern had been focused on central Virginia, where a flood watch was in effect. The sudden jump in flood threat 50 or so miles to the north, in the late afternoon, came as a surprise.

A stalled front, which was located to the south of D.C. during the early morning, surged northward. By late afternoon, it had shifted into the Baltimore region. This placed Northern Virginia, the District and central Maryland on the unstable and humid side of the front, with winds from the south.

The total available moisture content in the atmospheric column — as measured by the morning weather balloon in Sterling, Virginia — was around 2.2 inches, nearly a record for the day in July. The atmosphere became more and more primed through the afternoon, south of the front, for storms to wring out this excessive moisture.

As the image below shows, at around 5 p.m., a narrow band of thunderstorms erupted along the southern border of the front, across northern Fairfax County, lower Montgomery County, northern D.C. and northern Prince George’s County. The complex, as a whole, remained nearly stationary for a couple hours. But individual storm cells repeatedly developed on the western side and moved through the larger cloud complex, toward the east, generating a succession of cells passing over the same areas. This phenomenon is termed “echo training.”

Radar during flash flooding early Saturday evening in the D.C. area. The green polygons signify areas under flash flood warnings. (Radarscope)

Rain totals very rapidly added up across this narrow zone, as half a dozen flash flood warnings were rushed out.

More downpours amid a historically humid summer

Saturday’s downpours occurred amid an unusually humid and stormy stretch that began in June. It has been the second-most humid summer on record and flooding thunderstorms have occurred repeatedly.

When the Weather Service issued a flood watch for the region late Saturday afternoon, it marked the 16th since May, which is the most on record during a three-month period. The D.C. area has seen more flood watches than any other major city this year.

Because of human-caused climate change, which speeds up evaporation, the amount of available moisture in the air has increased in recent decades. A measure of atmospheric moisture, known as precipitable water, has steadily increased since the late the 1970s. The increased moisture raises the ceiling for the amount and intensity of rainfall.

After another humid and potentially stormy day on Sunday, the region expects a reprieve early next week as cooler and drier air settles in, at least briefly.

Dan Stillman, Ben Noll and Ian Livingston contributed to this report.