A long duration, heavy rain event is bearing down on the D.C. region. Slow-moving showers and thunderstorms got an early start on Friday and will continue through at least Saturday morning. Flash flood warnings have already been posted and more are possible in the coming hours.

Rainfall totals will range from 2 to 4 inches, though it’s possible isolated areas could see around 6 inches when this is all said and done. We’re also seeing the chance of intense wind gusts, which could lead to power outages.

Please see PM Update for details through the evening. 


3:30 p.m. update

Another band of heavy rain has set up in the Beltway — this time it’s focusing on some of the west and south suburbs that didn’t get much earlier this morning.

Here are some rainfall totals through 3 p.m. Friday. Some areas have received more than three inches of rain while their neighbors one town over have been dry. This is a very interesting event!

The Washington Post (downtown) — 0.99 inches
National Airport — 0.54 inches
Arlington — 1.2 inches
Alexandria — 1.48 inches
Dulles Airport — 0.07 inches (really!!)
Silver Spring — 2.39 inches
GLenmont — 5.07 inches
Frederick — 1.08 inches
Laurel — 1.98 inches
Bowie — 0.54 inches
Annapolis — 0.12 inches
Baltimore — 0.45 inches
BWI — 0.63 inches

As much as 6 inches of rain is expected to fall in D.C. through July 29. (The Washington Post)

1:08 p.m. update

Torrential bands of rain have set up from the District north through central Maryland late this morning, leading – already – to spotty 3- to 4-inch rain totals and numerous flash flood warnings. This graphic shows where the heaviest rain has fallen so far on Friday.


Clearly this is an earlier start than we had anticipated, and the main storm system (cyclone) has not yet even developed over the Mid-Atlantic. The graphic below illustrates a reason for this early arrival. A nearly stationary front is draped across the Mason Dixon Line. A small area of low pressure has developed just south of this boundary, within a deeply pooled air mass of moist air (precipitable water values of 1.9 to 2 inches). There is likely a zone where the air streaming into the low, on its north side, is getting “pinched” or strongly converged along and ahead of the front (purple arrows).


(National Weather Service)

The convergence is forcing the air to rise vigorously — stoked further by a generous amount of buoyant (unstable) air fueling thunderstorms. With very weak winds aloft, multiple bands of storms are drifting only very slowly eastward. The result is “training” or repeat passage of convective cells over the same locations (the so called “rain train effect”). Strong convergence of air is continued to continue through the afternoon, and become more widespread and dynamical in nature as the upper level disturbance in the jet stream arrives overnight.


Tourists walk near the U.S. Capitol building during a heavy rain, on July 28, 2017 in Washington, D.C. (Photo by Oliver Contreras/For The Washington Post)

Timeline

The worst of the rain looks like it will come through between 5 p.m. tonight and 5 a.m. Saturday. It’s starting to look more likely that rain will steadily taper off Saturday afternoon, but showers are still possible into Saturday evening.

  • Friday afternoon and evening: Widespread, slow-moving showers and storms develop, likely to produce very heavy rain.
  • Friday night: Waves of thunderstorms with heavy downpours. This will be the worst of it.
  • Saturday: Windy with periods of rain, possibly heavy at times. Rain should taper off through the afternoon.
  • Saturday night: Windy with intermittent showers possible — especially east and northeast of Washington.
  • Sunday morning: Rain should be over, becoming partly sunny and breezy.

Forecast rain amounts through Sunday morning

  • National Weather Service: 2 to 4 inches (3.8 inches in Washington)
  • GFS model: 2 to 5 inches (3.3 inches in Washington)
  • NAM model: 1 to 7 inches (4.2 inches in Washington)
  • Canadian model: 2 to 4 inches (4.3 inches in Washington)
  • European model: 1 to 5 inches (1.6 inches in Washington)


(models.weatherbell.com)

This is a very unusual storm system to develop in late July — in some ways resembling a winter Nor’easter. But because it is the middle of summer and the air is much more humid, this storm system will be able to generate significantly more precipitation than a winter storm.

Heavy storms may line up along a corridor and hit the same areas repeatedly — a worrisome phenomenon known as training. Areas that experience training will be most prone to flooding.

Within moments a flash flood can unleash waters into low roads, parking lots and basements. This is how a flash flood occurs and what you need to do to stay safe. (Claritza Jimenez/The Washington Post)

On Thursday afternoon, National Weather Service issued a flash flood watch for the region between Friday afternoon and Saturday afternoon, noting the potential for three or more inches of rain. We expect flash flooding will occur in the areas that get the heaviest rain.

Jason Samenow, Wes Junker and Jeff Halverson contributed to this report.