How flooding rain deluged District and northern suburbs Saturday night

Hints of a derecho-climate change link, ten years after 2012 storm

The zones between Silver Spring and Hyattsville and Rockville and Derwood, where up to 5 to 7 inches of rain fell, recorded the most rain. But a large swath of southern Montgomery County, the northern sections of the District and northern Prince George’s County saw at least 2 to 4 inches, falling over the course of several hours.

In addition to the flooding, a severe thunderstorm that swept from Potomac to Rockville between 7 and 7:30 p.m. Saturday downed trees, including one on a home, displacing residents.

In the wake of the storms, there were nearly 17,000 power outages in Maryland on Sunday morning, about 12,000 of them in Montgomery County.

In all, the National Weather Service received 31 reports of flooding Saturday night into early Sunday and 10 reports of wind damage, mainly from downed trees.

Here is a summary of the flood reports:

• The road connecting the Beltway and Clara Barton Parkway was blocked because of high water.
• Rock Creek Parkway, Potomac Parkway and Beach Drive in northwest Washington were closed because of flooding. Numerous vehicles were stranded in high water.
• A stream gauge on the northwest branch of the Anacostia River shot up 6 feet in one hour near Hyattsville. A stream gauge just to the east in Brentwood posted a rise of 7.2 feet in 50 minutes.
• A neighborhood in Hyattsville near the intersection of 23rd Avenue and Sheridan Street was flooded with “multiple people trapped or displaced,” according to the Weather Service.
• Near Takoma Park, East-West Highway was closed at Riggs Road as Sligo Creek overflowed. A vehicle was stranded near the intersection of New Hampshire Avenue and Sligo Creek Parkway.

• Just east of Silver Spring in Four Corners, three vehicles were stranded.
• Families were displaced in Silver Spring because of flooded basements and electrical hazards.
• In Chevy Case, Beach Drive was shut down because of Rock Creek overflowing between Connecticut Avenue and Kensington Parkway.
• Near Aspen Hill, a portion of MD-28 was closed because of overflowing water from Rock Creek, and there was a water rescue along Village Lane and Rippling Brook Drive.

Pete Piringer, spokesman for Montgomery County Fire and Rescue, tweeted that emergency crews responded to 449 incidents during the event.

Remarkably, while areas north of downtown Washington were deluged, very little rain fell to the south. Reagan National Airport picked up just 0.53 inches, while Dulles Airport reported nothing measurable.

What caused the flooding

The torrential rain was triggered by a very slow-moving front pushing southward across the region. The Weather Service issued a flood watch in advance, indicating the potential for several inches of rain. But amounts exceeded projections in some areas.

It initially appeared that the region might avoid the worst during the late-afternoon and early-evening hours Saturday, when the rain was initially predicted to start. Much of the storm activity was focused northwest of the area — near Frederick, Md., and to the west.

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But as the front sagged south after about 7 p.m., storms began to erupt in Montgomery County and pass from west to east repeatedly over the same areas, a phenomenon known as training.

The storms were able to tap into tremendous moisture. It had been a hot, humid summer day, and precipitable water, a measure of the moisture levels from high in the sky down to the ground, was excessive.

The high-resolution NAM model simulated precipitable water values between 2 and 2.5 inches over the region — in record territory for early July.

In a special bulletin issued at 11:24 p.m. Saturday, the Weather Service wrote that rainfall rates could “be as high as 2.5 inches/hour.”

A few other factors intensified the storminess. The Weather Service’s bulletin mentioned approaching “shortwave energy” from the Ohio Valley that helped sustain the storms deep into the night. The storms also fed upon one another. As one group of storms passed their cool exhaust or outflow would help destabilize the atmosphere for additional storms.

Finally, the rainfall was probably boosted by human-caused climate change. There is well-documented increase in the intensity of the most extreme heavy rain events in the eastern United States, fueled by an atmosphere that is becoming warmer and more humid.

Climate change has increased humidity in D.C., making it feel even hotter

This extreme precipitation event follows several others in recent years, including two historic floods in Ellicott City, Md., in 2016 and 2018 and one of the most exceptional downpours on record in Washington in July 2019, when 3.44 inches poured down in just one hour.

Such events are likely to become more frequent and intense in the coming decades as temperatures continue to rise.