6:15 p.m. — Storms exit D.C. area, but are blasting Southern Maryland
As storms have departed the immediate D.C. area, the National Weather Service discontinued the severe thunderstorm watch. However, very heavy storms are passing through the southern half of Southern Maryland which is experiencing torrential rain and strong winds. Severe thunderstorm and flash flood warnings are affecting some areas where two to three inches have fallen and another inch or two is possible. Storms could affect this area for at least another 30 minutes.
This will be the last update in this article. Stay tuned our PM Update for the forecast for the rest of this evening into tomorrow, publishing at 6:30 p.m.
4:50 p.m. — Storms sweeping east and southeast of Beltway; intense storm approaching Fredericksburg
The storms currently stretch from near Annapolis southwest to Waldorf; they have largely exited the District. While the storms contain very heavy rain, lots of lightning and some strong winds, they are mostly below severe limits with wind gusts of 30 to 50 mph (severe is over 58 mph).
The exception is toward southwest Charles County as well as the zone just west of Fredericksburg where the storms are rather intense. In fact, the storms about to enter Fredericksburg could unleash gusts up to 70 mph.
Severe Thunderstorm Warning including Fredericksburg VA, Lake of the Woods VA and Falmouth VA until 5:15 PM EDT. This storm will contain wind gusts to 70 MPH! pic.twitter.com/ejOIuJLqXG
These storms will continue pushing east and southeast over the next hour, passing through southern Maryland and exiting most locations by around 6 p.m.
4:15 p.m. — Storms span from Laurel to Quantico along I-95
A vigorous line of summer storms stretches from near South Laurel through the District and then south through Dale City to around Quantico. Some of the heaviest downpours and strongest winds around Landover, just south of Burke and west of Triangle.
Earlier, winds gusted to 67 mph in Warrenton
All of this activity will cross Interstate 95 and emerge east of the Beltway over the next 30 minutes. The heaviest storms may parallel Route 50 in Maryland and also affect the western parts of Charles County.
3:45 p.m. — Severe storm warning for downtown Washington and areas just north and east until 4:30 p.m.
Storms sweeping inside the Beltway are intense enough for a severe thunderstorm warning for southern Montgomery County, the District (from downtown areas northward) and northern and central Prince George’s County. In addition to torrential rain and lightning, some wind gusts could approach 60 mph in this zone. The strongest winds are currently near Chevy Chase headed toward Silver Spring.
3:40 p.m. — Heavy storms from Rockville to Manassas closing in on Beltway area
Heavy storms with strong winds stretch from Rockville to Reston to Centreville to Manassas to Nokesville from north to south. There currently aren’t warnings in effect for these storms but they contain lightning, very heavy rain and wind gusts probably in the 30- to 50- mph range. They’ll move inside the Beltway from northwest to southeast over the next 15 to 20 minutes and push into areas east of the Beltway between about 4 and 4:15 p.m.
3:10 p.m. — Intense storms stretch from Germantown to Warrenton
Storms have consolidated into a solid line from roughly Germantown in Maryland to Warrenton in Virginia. The strongest activity just swept through areas east of Leesburg toward Poolesville where radar showed the potential for some very strong wind gusts; the storms on the southern end of the line around Warrenton were showing signs of intensification, prompting a severe thunderstorm warning for central Fauquier County and western Prince William County until 3:45 p.m.
At their current rate of motion, storms should reach the Interstate 270 corridor between 3:15 and 3:30 p.m and the Beltway area and Interstate 95 by around 3:45 to 4 p.m.
2:30 p.m. — Severe storm warning until 3:15 p.m. includes Leesburg and Gaithersburg
Radar shows strong to severe storms around Frederick, Md. continuing to the southwest toward Purcellville in Loudoun County. Somewhat weaker storms trail to the southwest through northern Fauquier County. A severe thunderstorm warning covers northern Loudoun and western Montgomery counties until 3:15 p.m. This storm could produce wind gusts up to 60 mph and some small hail as it sweeps eastward at 40 mph.
It looks like these storms will reach the Beltway area between about 3:30 and 4 p.m.
1:55 p.m. — Severe thunderstorm watch until 10 p.m.
As showers and storms reach the Interstate 81 corridor, the National Weather Service has issued a severe thunderstorm watch for the Washington-Baltimore area until 10 p.m.; the watch also includes Roanoke, Richmond and Philadelphia — a total population of more than 20 million people.
“Damaging winds are the primary hazard,” the watch states, although torrential rain and lightning are also a threat.
Remember that a severe thunderstorm watch means conditions are favorable for severe storms, but a not a guarantee. On the other hand, if a severe thunderstorm warning is issued for your location it means a dangerous storm is imminent and you should seek shelter immediately.
Original article from 1:30 p.m.
Monday will probably mark the eighth day in a row with highs in the 90s in Washington, but a cold front barging into the region will put an end to that streak. This front will set off widespread showers and storms as we transition to this less hot air mass.
The heaviest storms will sweep through swiftly, arriving in our western areas by 2-3 p.m., right around the Beltway close to 3-4 p.m. and pushing over the Chesapeake Bay by 5 or 6 p.m. — although this timing could shift slightly. There could be a weaker, trailing line of showers and storms closer to sunset that exits by 10 or 11 p.m.
The afternoon round of storms could produce heavy downpours, lightning and a few bursts of damaging winds. The National Weather Service, which placed the area under a level 2 out of 5 risk for severe weather, issued a bulletin indicating it is “likely” (80 percent chance) to issue a severe thunderstorm watch.
A severe thunderstorm watch has already been issued for much of the Northeast from the New York City area through northern Maine until 8 p.m.
The storm potential Monday afternoon and evening is predicated on the arrival of a cold front from the northwest, intersecting an unstable and humid air mass along and ahead of it. The surface forecast chart below shows the position of the front at 8 p.m.:
Meanwhile, in the middle atmosphere, a belt of strong winds will be oriented from Pennsylvania to Maine, and, parallel to the front, will shift toward the northeast.
Storms have begun to develop along the crest of the Appalachians to our west; these storms will then drift toward Interstate 95 during the mid to late afternoon.
Given some sunshine and plentiful low-level moisture, we expect the atmosphere to destabilize to modest levels by midafternoon. This will energize and enlarge cloud updrafts. However, thick clouds drifting over the area from storms further northwest may prevent the atmosphere from reaching full destabilization Monday afternoon.
With moderate instability, the degree of storm intensity and organization is determined by wind shear, which is the increase in wind speed with altitude. Wind shear levels as measured by weather balloon Monday morning at Dulles Airport did not reveal exceptional levels, only around 20 mph (18 knots). With the approaching ribbon of faster wind aloft, however, we expect these values to climb to closer to 40 mph (30-35 knots) through the afternoon.
The image below shows the forecast wind shear values at 5 p.m. The D.C. region will lie along the southern edge of higher wind shear values to the north; the core of strongest shear will be well to our north. Such strong wind shear values in an unstable atmosphere may trigger isolated supercell thunderstorms, with the best chance for damaging wind, tornadoes and large hail.
In the Washington region, we expect torrential rain, lightning and a few pockets of strong to damaging wind or downbursts. The activity will be progressive, so we don’t expect storm cells passing repeatedly over the same area or training. But due to the intense updrafts and abundant low level moisture, some spots may pick up a quick one to two inches of rain — enough to trigger isolated flash flooding in areas that have received large amounts of rain this month.