6:30 p.m. - Forecast for the rest of the evening and Tuesday

There are no longer any warnings for severe storms in the region although one storm with downpours and lightning is pushing through Arlington and Alexandria and the southern half of the District (see 6:20 p.m. update) while a separate complex of showers and storms east of the Beltway pushes toward the Bay.

Through sunset, showers and storms should generally weaken and/or exit to the east. Overnight it’s partly to mostly cloudy, with lows ranging from about 66 to 72. A patch or two of fog may develop after midnight. Winds are light.

For tomorrow, it’s warm and muggy from the start. There’s a bit of haze to the air along with partly sunny skies. Clouds thicken during peak heating and another bout of showers and storms is possible, especially in the afternoon and evening. A few storms could turn strong to severe or produce heavy rain. Before any storms, highs rise to the mid-and-upper 80s most spots. Winds are variable around 10 mph.

This will be the last update of the evening unless new severe storms develop in the immediate area.

Here are a couple nice views of the storms this evening:

6:20 p.m. - Intense storm developing over Annandale and Falls Church, headed toward south Arlington and Alexandria, and eventually Nationals Park

Radar shows a storm has flared up over Annadale, right along the west branch of the Beltway and is headed east-southeast into Falls Church, south Arlington, and Alexandria and eventually towards Nationals Park and Capitol Hill. It could briefly delay the start of the Nationals game but should only last about 30 minutes in any location. Heavy rain, lightning, and gusty winds likely with this storm.

6:05 p.m. - Second wave of storms strengthening some east of Washington. New warning for southern Prince George’s and northern Charles County

Scattered storms which passed through the District and parts of our southwest suburbs have congealed into a line from the near the intersection of Rt. 50 and Beltway, south to near Waldorf and St. Charles, where a severe thunderstorm warning is in effect. Just west of Waldorf and St. Charles, radar indicates the storm may be producing hail and some very strong wind gusts. This line is moving east into southeast Prince George’s County, Calvert County, and southern Anne Arundel County.

5:10 p.m. - New storms developing west and southwest of Washington and likely to affect parts of immediate area

While today’s most intense storms exit our eastern areas and head across the Chesapeake Bay, some new scattered storms have developed west and southwest of the District.

So far these storms have not become severe but it’s not out of the question they intensify a bit. These hit-or-miss storms should come through the immediate area and our southern suburbs over the next one to two hours and, at the very least, will produce some downpours and lightning.

4:55 p.m. - Severe thunderstorm warning Bowie to Annapolis until 5:30 p.m.

An intense storm with heavy rain, lightning, and perhaps some pockets of damaging winds is pushing east along and just south of Rt. 50. It’s currently around Bowie and will continue east, passing through Annapolis over the next 30 minutes or so. The strongest part of the storm should remain south of the highway.

4:45 p.m. - Three severe storms along and east of Interstate 95

Storms so far are pretty scattered but those that have formed are packing a bit of a punch. We have three active severe thunderstorms warnings at the moment:

  • A severe storm which produced some small hail in northwest Washington and passed through Takoma Park has exited the city. But is now passing through Landover and headed into Bowie. It may contain small hail and isolated pockets of damaging winds, in addition to torrential rain and flooding. The warning expires at 5 p.m.
  • North of town, a severe storm is positioned just east of Columbia and is hitting Elkridge. It should move into northern Anne Arundel County and hit Glen Burnie around 5 p.m. It could also contain some small hail and damaging winds.
  • Southeast of town, severe storms are passing through central St. Mary’s and southern Calvert counties where warnings are in effect until between 5 and 5:15 p.m. The storm town near Leonardtown, moving east, may producing damaging winds and hail.

4:20 p.m- Severe thunderstorm warning for the northern District through Silver Spring and east to Landover until 5 p.m.

4:07 p.m. - Severe thunderstorm watch issued for immediate Washington area and points east

As storms begin erupting in the Interstate 95 corridor and points east, the National Weather Service has issued a severe thunderstorm watch until 11 p.m. Remember that a watch means conditions are favorable for severe storms, but not a guarantee - and that you should stay alert. If a severe thunderstorm warning is issued, however, it means a severe storm is imminent for your location and that you should seek shelter.

The watch includes Montgomery, Fairfax, and Prince William counties, as well as the District, but not locations to the west.

2:40 p.m. - 40 percent chance of a severe thunderstorm watch

Storms, so far, have been slow to develop (a couple of isolated cells have formed), but models simulate the most activity firing up between 3 and 5 p.m. We’ll monitor what happens and will be able to better assess whether storm coverage will be widespread or, like yesterday, more isolated. Right now, we’d favor something in between.

The Weather Service indicates there’s a 40 percent chance it will issue a severe thunderstorm watch for the area, writing: “A few strong to severe storms will initially develop over northern VA and central MD this afternoon, tracking eastward toward the coast. The strongest cells may produce locally damaging wind gusts. A watch is possible.”

1:50 p.m. - Flash flood watch canceled for immediate D.C. area but continues for northern Maryland

The National Weather Service canceled the flash flood watch for Fairfax, Montgomery, and Prince George’s counties, as well as the District, as atmospheric moisture levels do not support it. However, it still predicts numerous storms to develop into the evening, and the flash flood watch remains in effect in northern Maryland until 10 p.m.

Original post from 12:55 p.m.

Heat and humidity are building in the Washington region, and thunderstorms are likely in the region between midafternoon and mid-evening Monday — some potentially coinciding with the commute home.

A few thunderstorms could be severe with damaging winds and hail, in addition to heavy rain and lightning. Some places could get hit by multiple storms in rapid succession, presenting a risk of isolated flash flooding.

We do not expect widespread flooding but, rather, the chance for a few pockets of high water in any areas hit by multiple heavy storms. Poor drainage areas as well as streams and creeks are most susceptible to this kind of flooding.

While we expect numerous storms to develop, especially in the late afternoon and early evening hours, there’s some uncertainty as to just how widespread they will be. The greatest coverage area of the storms may end up from around Washington northward, with somewhat more hit-and-miss activity to the south.


Lightning over Washington in September. (Kevin Ambrose)

Storm dashboard


(The Washington Post/The Washington Post)

Approximate arrival time:

  • 2:30 to 4:30 p.m. in western areas.
  • 3:30 to 5:30 p.m. in immediate area, including the Capital Beltway.
  • 4:30 to 6:30 p.m. in areas east of Interstate 95.

More than one round of storms possible.

All clear: after 10 p.m.

Storm duration: 30 to 45 minutes or so, but more than one round possible.

Chance of measurable rainfall in any location: 65 percent.

Storm motion: west to east.

Likely storm effects: heavy rain, lightning, gusty winds.

Possible storm effects: damaging wind gusts, flash flooding, hail, isolated tornado.

Rainfall potential: Highly variable; average may be 0.5 inches, but some areas could see two to three inches, while some areas could end up with little rain.

Discussion

Monday afternoon and evening are looking active in terms of thunderstorm coverage. The National Weather Service Storm Prediction Center has placed the Mid-Atlantic corridor in a slight risk (level 2 out of 5) zone for severe storms for later Monday afternoon and evening. The threat is mainly for damaging wind gusts (15 percent probability within 25 miles of a location) and secondarily for large hail (5 percent chance).


Risk of severe storms Monday. (National Weather Service Storm Prediction Center)

Mostly sunny skies are enabling the atmosphere to significantly destabilize. This will allow plenty of buoyant cloud energy to build by mid- to late afternoon. Additionally, low-level moisture is on the increase courtesy of south-southwesterly winds. This increases prospects for locally torrential rain within storms.

At the surface, a cold front lurks just to our north (near the Mason-Dixon Line), and a weak wave of low pressure may scoot along and to the south of this front during the next six to 12 hours. With air converging along the frontal boundary and low pressure, there may be enough triggers to support widespread storm development.


Forecast weather map Monday evening. (National Weather Service)

The one fly in the ointment is the possibility that surface winds could veer even more from the west as the afternoon goes on. If this happens, convective coverage would be more limited, because of drier air surging down the eastern slopes of the Appalachians.

Wind shear (the increase in wind speed with altitude) will play an important role in storm behavior Monday. Shear causes cloud updrafts to tilt off-vertical, which increases their intensity, and also enables stronger cloud downdrafts to evolve. Overall, moderate shear can increase storm strength and duration.


HRRR model simulated radar between 3 and 10 p.m. This is just a model. Actual storm placement and timing will vary.

Monday’s mode of wind shear favors multicellular-type clusters and short lines of storms, moving east off the mountains. Cells that become arc- or bow-shaped may harbor damaging wind gusts. Shear intensity should be decreasing through the afternoon, but if strong pockets of wind aloft linger, then short-lived areas of updraft rotation could develop. If one or more of these supercell-type storms emerge, large (more than one inch) hail or a brief tornado is a remote possibility.

The low- and mid-level winds aloft will also align parallel to the weather front. Whenever this happens and the boundary is stationary (as it is Monday), a process called “echo training” can develop. This involves repeated upwind formation of storm cells, and downwind passage, over the same locations. The heavy rains come in pulses, that may last for several hours. Thus, combined with the high humidity levels, there is a risk for some pockets of flash flooding Monday afternoon and evening.

High-resolution models such as the NAM and HRRR all portray plenty of intense storm cells, coming in multiple waves or batches Monday afternoon and evening.

It’s possible that a severe thunderstorm watch is issued for the region. We will monitor the situation through the day and provide updates.