4:00 p.m. – Storms exiting immediate D.C. area to southeast. Another round possible Wednesday.

The worst of the showers and storms have now mostly pushed east of Interstate 95 and well south of the Beltway. They are producing some heavy rain but are not severe. They will pass through Southern Maryland over the next hour or two, where they will unleash some downpours, lightning and perhaps a few strong wind gusts.

As today’s storms pushed through the immediate D.C. area, they generally dispersed about 0.25 to 0.75 inches of rain. In a narrow stripe from Leesburg to McLean to Arlington/Alexandria and into Clinton and Brandywine, 0.75 to 1.5 inches of rain were estimated by radar.

Although several severe thunderstorm warnings were issued for the threat of damaging winds, we did not see any actual reports.

On Wednesday, we may well have another round of storms. For more information on that forecast, see our PM Update, which should be published by 5 p.m. or so at this link: Capital Weather Gang posts

3:40 p.m. – Southern areas now deal with heaviest storms

Heavy thunderstorms have now edged south of the Beltway and the most intense activity is generally south of both Interstate 66 and Route 50. Radar shows some rather vigorous storms about to ram into Interstate 95 from Lorton to Dumfries, which are sure to slow the beginning of the commute. Between 4 and 4:30 p.m., this activity should push into Southern Maryland.

To the north, we’ll just have scattered light rain showers for the next hour or two and brightening skies.

3:15 p.m. – Temperatures tumble in rain-cooled air

As the storms passed through the area, temperatures have dropped 10 to 15 degrees. In downtown Washington, at our weather station on the roof of The Washington Post, the temperature fell from 92 to 78 degrees.


Temperatures and dew points at The Washington Post on June 19. (Weather Underground)

3 p.m. – Widespread showers and storms with pockets of heavy rain

We no longer have active severe thunderstorm warnings over the region, but much of the Beltway is covered in rain, which is locally heavy. Radar also shows heavy storms around Fairfax, Annandale and Burke, and to the west from Sterling to around Haymarket. All of this activity is slowly sliding southeastward and should sink south of the Beltway between 4 and 5 p.m.

Radar indicates some areas have witnessed very heavy rainfall with a stripe of at least an inch from near Tysons Corner through Arlington across the Potomac into Clinton, Md.

If you live north of the Beltway, you’ve seen the worst of today’s storms already, with just some lingering showers for the next hour or two.

2:40 p.m. – Severe thunderstorm warning from Middleburg to Centreville through 3:00 p.m.

An area of strong to severe storms is moving into southeast Loudoun, and northwest Fauquier counties and may advance southeast into western Prince William and southwest Fairfax County over the next 30 minutes. Some pockets of very strong winds are possible with these storms.

2:20 p.m. – Severe thunderstorm warning for District and nearby areas through 2:45 p.m.

A heavy thunderstorm with dangerous lightning and potentially damaging winds is moving into the District and areas just to the south, prompting a severe thunderstorm warning for D.C., eastern Fairfax County, and southwest Prince George’s County. Wind gusts up to 60 mph are possible.

The heaviest part of this storm is south of downtown Washington, and extends from southeast Arlington through Alexandria. It is headed toward Oxon Hill and Marlow Heights in Prince George’s County.

Seek shelter from this storm.

2 p.m. – Storms increasing in coverage and intensity

Over the past hour, storms have become more numerous across the metro region and also a bit stronger. The National Weather Service has not yet issued any severe thunderstorm warnings, but that cannot be ruled out over the next couple of hours.

Radar showed the heaviest and most concentrated storm activity northwest of the Beltway between western Loudoun County and areas just west of Interstate 270 in Montgomery County. All of this activity is progressing toward the southeast in the general direction of the Beltway and the District, where rain is just beginning to move in and should continue over the next hour or two.

The National Weather Service has increased the region’s severe storm threat level from marginal to slight so, in addition to heavy rain and lightning, we’ll need to watch for the possibility of some strong wind gusts developing in the strongest storms.

1 p.m. — Storms starting to erupt

Showers and thunderstorms, with locally heavy rain, have developed along and east of Interstate 270 and just east of the District. This activity is quickly advancing northwest to southeast. Expect additional showers and storms to flare up throughout the metro region over the next hour.

Original post from 12:30 p.m.

Washington is one of the hottest and most humid locations in the nation today and, as a cold front runs into this muggy air, scattered strong thunderstorms will erupt this afternoon into the early evening.

Storms are likely to contain dangerous lightning and heavy rains. Isolated pockets of damaging winds are possible.


Simulated radar from HRRR model through 7 p.m. This just gives a generalized idea of how storms may evolve. Actual results will vary.


Storm dashboard


Approximate arrival time for storms:

  • North of Beltway: Through 2 p.m.
  • Near Beltway and along Interstate 66 and Rt. 50: 1 to 3 p.m.
  • South of Beltway: 1:30 to 4 p.m.

Storm duration: 30 to 45 minutes

Chance of measurable rainfall in any location:

  • North of Beltway: 40 percent
  • Near Beltway: 50 percent
  • South of Beltway: 60 percent

Storm motion: Northwest to southeast

Likely storm effects: Heavy rain, lightning

Possible storm effects: Damaging wind gusts, flash flooding

Very small chance of: An isolated tornado

Rainfall potential: Average 0.25 to 0.5 inches, pockets of higher and lower


Rainfall projection from HRRR model. Notice how amounts are variable. The pattern of rainfall shown here is only a generalized idea, and localized amounts will vary significantly.


Discussion

Today’s forecast challenge is to anticipate scattered thunderstorms erupting along a weak and diffuse backdoor cold front moving from north to south during the afternoon. This cooler air mass will be plowing into a very hot, humid and unstable air mass in place over the greater D.C. region. The strength, movement and location of the front will prove challenging, and thus there is some uncertainty about where the best corridor of storminess will set up.

A very unstable atmosphere argues for a chance of isolated severe storms, although wind shear levels may not be strong enough for a more widespread severe threat. Any storm that develops could be plenty strong, with frequent lightning, small hail and strong wind gusts with the potential for a few wet microbursts. These are intense bursts of wind and rain from the cloud down to the ground that can cause trees to topple.

Additionally, the deep atmosphere is high in moisture content, and the “steering level” winds may align parallel with the front. This suggests the possibility of some training thunderstorms and a localized flash flood potential.

Finally, any time thunderstorms develop along a low-level frontal boundary, there is the potential for some of the “spin” along that front to consolidate into a brief (and generally weak, short-lived) tornado. This, however, does require a strong and long-lived updraft to help stretch and tighten the rotation. It remains to be seen if enough wind shear in higher levels of the atmosphere develops to support this type of updraft.

The National Weather Service Storm Prediction Center says that while a few instances of locally strong to damaging wind gusts could develop in storms, it is unlikely to issue a severe thunderstorm watch.


Lightning strike over the cannons at Manassas National Battlefield Park, June 8. (Joseph Gruber via Flickr)