2:55 p.m. — Severe thunderstorm watch issued until 9 p.m.
The National Weather Service has issued a severe thunderstorm watch for the entire D.C. region until 9 p.m. The watch also includes all of Delaware and expands into eastern Pennsylvania and parts of New Jersey.
A watch means conditions are favorable for storms but severe weather is not a sure thing. If severe weather is imminent or occurring for your specific location, a severe thunderstorm warning will be issued.
“Thunderstorm-related wind damage is the most probable risk, but some hail and a low tornado risk will exist as well,” the National Weather Service wrote.
Just before 3 p.m., radar showed scattered storms in parts of Loudoun, northern Montgomery and northern Fauquier counties. Storms are expected to become more numerous and may consolidate into a line by early evening.
We’ll post another update around 3:30 p.m. or sooner if storms become severe.
Original article from midday
A band of strong to locally severe thunderstorms is expected Thursday evening across the Washington region, bringing the risk of gusty to damaging winds, lightning and brief torrential downpours. It’s only the third day of meteorological summer, but the D.C. area finds itself in a warm, muggy pattern, supportive of organized thunderstorms and a soupy atmosphere.
Additional heavy thunderstorms will be possible Friday afternoon, too, but coverage and severity should be less than Thursday.
The National Weather Service Storm Prediction Center has included the region in a Level 2 out of 5 “slight risk” for severe weather, highlighting a stretch along the Interstate 95 corridor from Richmond to New York City, where there exists the greatest propensity for disruptive storms.
A flash flood watch is in effect for Montgomery and Frederick counties and locations to the east from 3 to 10 p.m. The Weather Service says that rainfall amounts may average only about a half-inch but that “locally higher amounts of 2 to 4 inches are possible in a short period of time.”
Timing: An approaching cold front will trigger thunderstorms during the late afternoon and evening, but the slow-moving nature of the front broadens the window within which storms may arrive. Our best estimate is as follows:
- Interstate 81 (Hagerstown/Winchester/Front Royal): 2 to 5 p.m.
- Route 15 (Frederick/Leesburg/Gainesville): 4 to 7 p.m.
- Interstate 95 (Baltimore/Washington/Fredericksburg): 6 to 9 p.m.
- Route 301 (Crofton/Upper Marlboro/Waldorf): 7 to 10 p.m.
Some more isolated, pop-up storms could flare up ahead of these windows. Storms may be weakening as they push east of D.C. after sunset, but gusty winds, lightning and heavy rainfall are still possible.
Main threat: Strong to locally damaging winds, with isolated gusts over 50 mph. The strongest winds will accompany downbursts, which are erratic and local in nature.
Secondary risks: Very heavy rainfall and pockets of street flooding, especially north and west of D.C. In addition, small hail and an isolated, brief tornado can’t be ruled out. The tornado risk, while small, would be maximized where any local boundaries such as a breeze off the Chesapeake Bay enhance low-level spin.
Discussion
We continue to monitor observations and model guidance that suggests the potential for a stormy late afternoon and evening. On Thursday morning, a warm front has been pushing north of the region, with surface winds now arriving from the south. However, the region remains socked under a fairly solid, low-level deck of clouds.
The figure below shows the bigger picture — the forecast weather map as of 8 p.m. Thursday. The main center of action is over Ohio, with weak low pressure along a frontal boundary, well to our west. A trough of surface pressure lies along the mountains. The clockwise circulation around high pressure centered near Bermuda is ushering in a warm, very humid airflow from the south.
Aloft, meanwhile, a ripple in the jet stream moves across the D.C. area, broadly increasing uplift and freshening the mid-level airflow from the southwest.
Most of the model guidance suggests that this combination of ingredients will set the stage for a line of convective showers and thunderstorms — congealing over the high terrain and then sweeping eastward in the time frame of 5 to 9 p.m., although some isolated storms could pop up earlier.
The forecast for locally severe storms hinges on whether the cloud cover breaks sufficiently to allow the sun to heat the surface air, thus destabilizing the atmosphere.
The morning weather balloon at Washington Dulles International Airport has a rather poor “lapse rate” — that is, the rate at which the temperature decreases with altitude is weak. Inherently, that does not portend a very unstable atmosphere. A lot of surface warmth will be needed to develop the buoyant energy needed for severe storms.
However, if pockets of surface heating can develop, there is plenty of wind shear (increase in winds with altitude) to organize storms into more intense, long-lived clusters within a larger line of storms. On a localized basis, these cells could rock-and-roll in terms of intense lightning, damaging wind gusts and small hail.
Another feature in the morning weather balloon is abundant moisture through a very deep layer of atmosphere. Dew point temperatures near the surface are quite elevated. Thus, even if storm cells do not reach severe levels, they could become efficient rainmakers.
With airflow uniformly from the southwest at all levels of the atmosphere (except the surface), and the main storm line similarly oriented, cells may track repeatedly over the same locations. Fairly brisk motion of the larger storm line may ultimately limit overall duration of heavy rain, but the tendency for repeat tracking or “training” could predispose a few areas to flash flooding. This may play out in particular to the northeast of the immediate D.C. region.
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Updates: Severe storms flaring up northwest of Washington - The Washington Post
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