Updated forecast for tonight and tomorrow

The afternoon storms were fairly well-behaved as they charged through the region, despite scattered reports of tree damage, mainly southwest and north of the Beltway. Many areas saw about 0.5 to 1.0 inches of rain (0.9 inches at Reagan National) and wind gusts between 30 and 50 mph — which is pretty typical for summer storms.

Now, we can enjoy some quieter weather for the next 24 hours, at least.

Tonight: With showers and storms clearing the region to the south and east, the night should be largely tranquil. There’s just a slight chance of a passing shower late night as skies remain partly cloudy. Lows settle to a range of mid-60s to low 70s as winds blow from the south and southwest around 5 to 10 mph. A few patches of fog may form late.

Thursday: The air mass is a bit cleaned out, so we should see less haze and more sun. High temperatures are in a similar zone as today’s readings, or near 90 and into the low 90s. Despite somewhat lower humidity, with dew points in the low-to-mid 60s or so, heat indexes approach the mid-90s in the afternoon. A line of showers and storms may approach from the northwest late in the day or during the evening, but for now it seems it will fall apart as it does so. Winds are from the west around 5 to 10 mph.

See below for our earlier storm coverage

Storm updates

5:30 p.m. — After gusty winds and heavy rain, worst of storms depart. Severe thunderstorm watch discontinued.

The most intense storms have reached the Chesapeake Bay and Delmarva. While there are some trailing thundershowers still to pass areas south and east of the Beltway, those should move off by around 7 p.m.

Elsewhere the storm threat is over and the severe thunderstorm watch is discontinued.

4:40 p.m. — Worst of storms pushing east of Beltway and Interstate 95 toward Annapolis. Drying out west of Interstate 270 and Fairfax County.

A line of storms stretches from east of Baltimore southwest through Bowie and into Clinton, Md., in Prince George’s County. The storms are racing eastward and are about to move into the Annapolis. Severe thunderstorm warnings cover much of Anne Arundel and southern Prince George’s County and are in effect until 5 p.m.

These storms will continue producing very heavy rain, lightning and some pockets of strong to damaging wind, before moving out over the Bay between 5 and 6 p.m.

We already see conditions clearing out in Montgomery and western Fairfax County and locations to the west. The severe thunderstorm watch has been discontinued for Howard, Montgomery, Loudoun and Fauquier counties and to the west.

4:20 p.m. — Storms sweeping through District, pointed at eastern suburbs. Warnings expanded south.

Storms continue racing east and the worst is passing just east of downtown Washington. The heaviest activity spans from Greenbelt to Camp Spring and is pushing east at 35 mph.

3:55 p.m. — Severe thunderstorm warning for much of the immediate metro area, including the District

Strong to severe storms with very heavy rain, lightning, pockets of strong winds, and even some small hail are moving through the immediate metro region, covering much of the Beltway, and expand to the southwest toward Centreville and to the northeast toward Columbia.

Some of the most intense activity is between Alexandria and downtown Washington, including Arlington, where there have been reports of small hail.

The storms are moving to the northeast around 30 mph.

3:30 p.m. — Storms stretch from Germantown to Centreville to Warrenton, pushing east-northeast. Warning for Fairfax County.

Storms presently form an arc from near Damascus and Germantown curling back to the southwest through northern and western Fairfax County into northern Fauquier County near Warrenton.

The storm over northern and western Fairfax County has begun intensifying and so the National Weather Service has issued a severe thunderstorm warning for much of the central and northern part of the county until 4:15 p.m. This includes much of the Interstate-66 corridor through Centreville and Fairfax. This storm could contain some damaging wind gusts and small hail.

This storm may move inside the Beltway and approach the District by around 4 p.m.

2:50 p.m. — Storms continue focused west and northwest of the Beltway

Storms now stretch along the Interstate 270 corridor northeast to just west of Baltimore while new storms are firing up in Virginia near Sterling and then scattered to the southwest in Fauquier County.

The storm near Sterling and Dulles has quickly intensified and may produce some damaging wind gusts as heads east-northeast into southwest Montgomery County through 3:30 p.m.

2:21 p.m. — Storms affecting air travel up and down East Coast

As storms erupt not only in the Washington region, but also through the Northeast, airport ground stops are in effect at BWI, Dulles, Newark, La Guardia, JFK and Boston airports. Delays are also mounting at Reagan National and Philadelphia airports.

A severe thunderstorm watch is also in effect for much of the Northeast from Wilmington and Philadelphia to Boston until 9 p.m.

2:07 p.m. — Multiple severe thunderstorm warnings northwest of the Beltway

The National Weather Service has issued four severe thunderstorm warnings to cover the area from southwest Montgomery County to Carroll County. In addition to heavy rain and lightning, storms in this zone may produce damaging wind gusts (up to 60 mph) and small to medium-sized hail.

1:57 p.m. — Severe thunderstorm watch issued until 8 p.m.

Given the storms already erupting in the region, some severe, and conditions favorable for more storms, a severe thunderstorm watch has been issued for the Washington region.

A watch means conditions are conducive for some storms to become severe, but not a guarantee — and that you should stay alert to weather conditions. If a severe thunderstorm warning is issued for your location, it means a severe storm is imminent and you should seek shelter.

1:55 p.m. — Intense storms developing west and northwest of the Beltway

Thunderstorms are rapidly developing in the western parts of our region. Scattered storms stretch from Centreville to Germantown. They are widespread and particularly intense to the northwest of Germantown toward Frederick, where severe thunderstorm warnings are in effect.

These storms are moving southwest to northeast.

Original post from 1:40 p.m.

Numerous showers and thunderstorms, a few of which could be locally severe, are likely this afternoon and evening as a vigorous upper-level weather system approaches from the west. Storms firing up early this afternoon out toward the mountains will be racing eastward, arriving in the immediate area by mid-to-late afternoon

Everyone is in play for this afternoon’s storms, which pose a risk of strong to perhaps damaging winds in a few locales, as well as spotty hail. If multiple storms pass repeatedly over the same area, then a few instances of flash flooding are possible. Storms should merge into a line and exit our area before sunset, although an isolated renegade shower or storm thereafter isn’t out of the question.

If you have plans for this afternoon, keep an eye to the sky, and be prepared to go indoors if/when storms threaten throughout this evening. Check back for frequent updates.

Storm dashboard

Approximate arrival time:

  • 1:30 to 3 p.m. in western areas.
  • 2:30 to 4:30 p.m. in immediate area, including the Capital Beltway.
  • 3 to 5 p.m. in areas east of Interstate 95.

All clear: Between 7 and 9 p.m. except for isolated pop-ups.

Storm duration: 30 to 45 minutes or so.

Chance of measurable rainfall in any location: 60 percent.

Storm motion: southwest to northeast.

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

Possible storm effects: damaging wind gusts, small hail.

Small chance of: large hail, isolated tornado, isolated flash flooding

Rainfall potential: highly variable. Locally up to 2 to 3 inches in heaviest storms.

Forecast discussion

The potential for widespread shower and thunderstorm activity is fairly substantial from midafternoon through evening. The combination of an unstable atmosphere, abundant moisture, strengthening winds in the middle atmosphere, an approaching front and jet stream energy aloft … could combine to produce a squall line of thunderstorms that will sweep across the Interstate 95 corridor.

The forecast surface map below, valid in the early evening, shows a cold front pushing across the Interstate 95 corridor. A weak area of low pressure may develop on this boundary, right over D.C., inducing further convergence of air, uplift and coverage of storms.

In the upper atmosphere, a potent disturbance called a “shortwave trough” will swing through, containing an intense pocket of spin energy (what we call “vorticity”), as shown below. This pocket will induce strong uplift of air as it approaches, and may help to organize a squall line.

The dip or trough in the jet flow will also strengthen mid-level winds over Washington, meaning the wind shear (increase in winds with altitude) will be enough to cause storm cells to intensify and become longer lived.

We expect cells to organize into arc-shaped segments, or even a single, long squall line, as suggested by this morning’s models (shown below).

There will be two hazards this afternoon from these storms: Damaging wind gusts and flash flooding. Not every location will experience damaging winds, but we expect that spotty downbursts containing wind gusts from 60 to 70 mph, will generate a few severe thunderstorm warnings. The Storm Prediction Center has placed our region in a slight risk zone for severe storms.

Additionally, with upper level winds blowing parallel to the front for several hours, storm cells may repeatedly initiate and pass over the same locations. This process, called cell training, may cause localized heavy rains and possibly some flash flooding. Some spots may pick up two to three inches in a short time. This poses a risk to locations that received excessive rain Tuesday, including Baltimore. The Weather Prediction Center, accordingly, has declared a slight risk of flash flooding for the Interstate 95 corridor.