* Flash flood watch until 6 a.m. Tuesday *

10 p.m. - Radar quiet around town, more showers possible later tonight and tomorrow

Radar has quieted down in the immediate metro area, where there are no longer any severe thunderstorm watches or warnings in effect. Storms earlier this evening were brief, but did produce a few scattered reports of trees down, including in D.C. and Takoma Park. We still have pockets of heavy rain moving through Frederick County, and we’re watching to see if an area of showers out west clips the area later this evening into the overnight as it tracks to the east-northeast.

We’re dealing with more shower chances into the morning Tuesday, including perhaps some thunder, especially south and east of Interstate 95. It may stay damp through the afternoon, although most of the rain should fall prior to noon. With all the clouds and rain around, we get a break in the heat, with highs rising to around 80. Winds are out of the north and northwest around 5 to 10 mph.

If heavy storms redevelop in the area, we’ll update this post. Otherwise, the above forecast holds until our next update early Tuesday morning.

8:25 p.m - Severe thunderstorm warning near and inside the Beltway, briefly heavy rain and damaging winds possible

Much of the immediate metro area near and inside the Beltway is under a severe thunderstorm warning until 8:45 p.m. (yellow boxes below), as a band of storms races through from southwest to northeast, producing briefly heavy rain the potential for damaging winds.

7:50 p.m. - Storms moving in from the southwest, could be briefly severe

More storms are moving into the area from the southwest. These have a history of wind damage with trees down, prompting the National Weather Service to issue a severe thunderstorm warning for a large section of our southern and southwest suburbs (yellow box below) until 8:30 p.m.

6:15 p.m. - Storms may be less numerous and intense than feared this evening but some still possible. Updated forecast.

While heavy storms have erupted to our northeast (from northeast Maryland through Philadelphia and New York City) and to our southwest (south of Charlottesville), we sit in a bit of dead zone where not much is happening.

We may see the showers and storms to the southwest expand northeastward, and reach our southern areas, but radar trends and short-term modeling are inconclusive as to whether they make it into the immediate metro area.

Irrespective of what happens with storms up through sunset, additional rounds of showers and perhaps some rumbles are likely through the night. This as a front settles through the area. Some of the rain that falls could be moderate to heavy. Lows fall mainly to the upper 60s and lower 70s. Winds are light from the north.

4:10 p.m. - A pause in storminess in the immediate area, but more expected after 6 p.m.

Radar shows a break from the storms inside the Beltway and in much of the immediate area except for a small storm cell over Aspen Hill in Montgomery County pushing northeast toward southern Howard County, and eventually perhaps into Columbia. This could put down some brief downpours and gusty winds as well as produce some lightning.

This lull in storminess, nicely timed for commuters, could continue until perhaps around between 6 and 8 p.m. when we see some more activity start to move into the region from southwest to northeast. Before that, we can’t rule out some more isolated pop-up showers and storms.

We’ll update next when storms move back into the region.

3:25 p.m. - Tornado warning discontinued for storm in southeast Frederick County and southwest Carroll County.

3:20 p.m. - Severe storms in southeast Montgomery County and southeast Frederick County, pushing northeast

Radar shows two intense storms north of Washington.

The first, warned for a possible tornado, is passing through Green Valley in southeast Frederick County and pushing east into northern Howard and southern Carroll county - very close to Mount Airy. So far, there’s been no confirmation of a tornado but radar indicates, at the very least, strong straight line winds. Residents in this area should seek shelter in an interior room, away from windows.

The second storm is between Kensington and Colesville in southeast Montgomery County and is headed toward Laurel in northern Prince George’s County, and eventually into northern Anne Arundel County, between 3:45 p.m. and 4:15 p.m. This storm, about to cross Interstate 95, could contain some locally damaging winds near Beltsville.

3:02 p.m. - Tornado warning from Urbana (in Frederick County) to Mount Airy until 3:30 p.m.

The National Weather Service says radar indicates rotation near Linganore-Bartonsville, or near Harry Grove Stadium on the south side of Frederick, moving northeast at 35 mph. New Market and Mount Airy in possible path. Whether tornado or not, some strong straight line winds likely in this zone. Seek shelter.

2:50 p.m. - Intense storms developing between Reston, Great Falls and Potomac and heading east

Radar shows storms flaring between Reston and Potomac. A severe thunderstorm warning has been issued for this area of storms as pockets of damaging winds could develop (although radar does not yet indicate them) given the very unstable atmosphere. At the very least, expect gusty winds, downpours, and frequent lightning are to be expected.

This area of storms is headed toward Interstate 270 corridor over the next 15 to 30 minutes and eventually Aspen Hill and Wheaton (by 3:15 to 3:30 p.m.).

2:22 p.m. - Severe thunderstorm watch issued for region through 10 p.m.

As storms begin erupting in our far western areas (western Loudoun and Frederick counties), the National Weather Service has issued a severe thunderstorm watch for the entire region. In fact, this watch covers much of the Mid-Atlantic and Northeast, spanning from roughly Charlottesville, Va. to Providence.

The Weather Service says thunderstorms in this zone could produce wind gusts up to 70 mph in addition to small hail, frequent lightning, and torrential rain. A tornado or two also cannot be ruled out.

A severe thunderstorm watch means conditions are favorable for storms to develop over a broad region, but it’s no guarantee any particular area will be hit. If, however, a severe thunderstorm warning is issued for your location, that means a potentially dangerous storm is imminent and you should seek shelter.

Original article from midday

A slow-moving cold front will collide with hot and exceptionally humid air this afternoon and tonight, setting off multiple rounds of showers and storms — potentially coinciding with the commute home.

Certain storms may be intense, unleashing strong to damaging winds and torrential rain in addition to frequent lightning. With some areas potentially being hit repeatedly by heavy storms, pockets of flash flooding could result. Trees and power lines could come down in localized bursts of strong winds, and some outages are possible. The soggy ground will make some trees particularly vulnerable.

This is a scenario in which not everyone will necessarily experience severe weather, but it would be wise to monitor conditions and have a plan to alter your routine if heavy storms develop. If you intend to be outdoors, identify a place where you could quickly and safely shelter. And if heavy storms intersect with your normal commute route home, consider waiting until the worst passes.

The heaviest storms are likely to come through intermittently between late afternoon and sunset, but on and off periods of rain, heavy at times, could continue overnight into Tuesday.

Storm dashboard


Approximate arrival time of the storm threat:

  • 2:30 to 4:30 p.m. in western areas
  • 3:30 to 5:30 p.m. in the immediate area, including the Capital Beltway
  • 4:00 to 5:30 p.m. in areas east of Interstate 95

Periods of rain and/or storms possible from Monday afternoon through Tuesday afternoon

All clear: The threat of severe storms wanes after sunset, but intermittent rain/thunder may not end until sometime Tuesday afternoon.

Storm duration: Multiple rounds are possible. Individual storms pass in 30 to 45 minutes.

Chance of measurable rainfall in any location: 80 percent.

Storm motion: Southwest to northeast.

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

Possible storm effects: Damaging winds, flash flooding, small hail.

Discussion

The heat wave of the past several days is not expected to “break” or end gracefully, without some stormy drama. As shown in the graphic below, a slow-moving cold front, presently over Pennsylvania, will sag slowly south over the next 24 hours, passing through the D.C. region.


Monday evening's weather map. (National Weather Service, adapted by Jeff Halverson)

One or more waves of low pressure will develop along the front, helping to import and converge low-level moisture over our region and force that air to ascend.

In the upper atmosphere, an energetic wave disturbance will transit the region from the west, causing air to rise, in tandem with air ascending along the front near the surface.

There is significant, deep moisture in the air mass, and that air mass will once again become unstable by midafternoon. Thus, all the ingredients for multiple rounds of showers and thunderstorms are in place.

There are twin threats late this afternoon and into the evening: the prospect of very heavy rain and severe thunderstorms.

The National Weather Service Weather Prediction Center has placed our region under a moderate risk of flash flooding. This is the result of the many forces causing very moist air to ascend and the storms that are likely to develop in response.

Additionally, the mid-level winds that will steer storm cells from southwest to northeast are aligned parallel to the slowly moving front. This is a classic setup for “training,” whereby multiple rounds of torrential rain may pass repeatedly over the same locations, in the space of a few hours.

Our region is under a Flash Flood Watch from late this afternoon until early tomorrow morning, as it will take a long time for the front to move south of the region and the upper air disturbance to move off to the northeast.

Many locations will receive 1 to 2 inches of rain, but some locations may very well experience 3 to 4 inches in a two- to three-hour period, enough to cause roadway problems coinciding with the rush hour. However, moderately heavy rain may continue through the night, contributing an additional 0.5 to 1 inches in some spots but falling over a longer period.

The graphic below illustrates the likely nature of storms later today, showing multiple waves or bands of activity developing ahead of the front, in the very unstable and humid air mass.


Simulated radar from the HRRR model at 4 p.m. showing multiple areas of storms approaching from the west.

The Weather Service’s Storm Prediction Center has placed the D.C. region in a slight risk zone for severe storms, and the threat is mainly for damaging wind gusts. The threat is operative mainly from 3 to 9 p.m. this evening.


Risk zones for severe weather on Monday. (National Weather Service)

There is enough wind shear (increase in flow speed with altitude) that storms will organize into more coherent clusters and short lines, what we call a “multicellular” mode, which is particularly efficient for generating heavy rain.

The combination of shear and enhanced dynamics in the atmosphere could lead to isolated to scattered pockets of damaging wind, called downbursts. Winds in these blasts often momentarily reach 60-80 mph. With soils becoming saturated by heavy rain, some trees are likely to come down.

As always, there remains the threat of frequent lightning and small hail.

Capital Weather Gang will be keeping a close eye on the situation as it unfolds later today and tonight.