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Storms with damaging winds possible in DC area late Thursday – The Washington Post

* Severe thunderstorm watch until 10 p.m. | Wind advisory until 8 p.m. *
4:40 p.m. — Radar is quiet but showers and storms to increase with time
Aside from some passing showers west and southeast of Washington, very little rain has developed over the region so far. Nevertheless, it has been quite windy, with frequent gusts over 40 mph.
Into this evening, we do expect to see showers and storms increase. Any that do, could produce locally damaging winds and has a slight chance to produce a tornado.
We still think the best chance of showers and storms, which could be locally intense, will come when the cold front comes through between roughly 8 p.m. and midnight.
2:10 p.m. — Severe thunderstorm watch issued until 10 p.m.; a tornado or two possible
Anticipating the storms through this evening, the National Weather Service has issued a severe thunderstorm watch for the entire region through 10 p.m.
“Thunderstorms will continue to increase in coverage and intensity this afternoon and spread across the watch area,” writes the National Weather Service Storm Prediction Center. “The strongest cells and lines will pose a risk of damaging wind gusts and a tornado or two.”
In the D.C. area, we may see gusty showers and storms start to increase around 5 p.m. and continue intermittently into the evening. They may be hit or miss.
A severe thunderstorm watch means conditions are favorable for severe storms, but not a guarantee and that you should stay alert. If a severe thunderstorm (or tornado) warning is issued, it means a storm is imminent for your location and you should seek shelter immediately.
A severe thunderstorm watch has been issued for parts of District of Columbia, Maryland, New York, Pennsylvania, Virginia and West Virginia until 10 PM EDT
The Weather Service also increased the region’s storm risk level from “slight” (level 2 out of 5) to “enhanced” (level 3).
To our south, developing storms have already resulted in tornado warnings on the west and north side of Richmond. At this point, we have not seen reports of a confirmed tornado or damage.
Note that while our discussion below indicates the most intense storms are probable later this evening when the cold front passes, showers and storms developing ahead of it this afternoon and evening could be severe and produce damaging wind gusts and perhaps a brief tornado.
We will post updates if severe weather moves into the area.
Original article from 1:20 p.m.
After an outbreak of severe weather in the South on Wednesday into early Thursday, the cold front responsible for the dangerous storms is charging toward the East Coast. As the front encounters mild, humid air surging northward, strong to severe thunderstorms may erupt in the Washington region, mostly this evening and tonight.
The Storm Prediction Center (SPC) has placed much of the eastern United States in a slight risk zone for severe thunderstorms. This is Level 2 out of 5 on the severity threat scale.
Storm threat shifts to East Coast after deadly tornadoes hit the South
The main threat with any storms will be damaging winds, although a short-lived tornado or two isn’t out of the question. It’s possible that the showers and storm pass through parts of the area without much fanfare. Although severe storms aren’t a sure thing, it would be wise to remain weather aware through late tonight.
Ahead of any thunderstorms, strong winds from the south — gusting up to 50 mph — have prompted a wind advisory for the region until 8 p.m.
“Gusty winds could blow around unsecured objects,” the Weather Service writes. “Isolated tree damage and a few power outages may result.”
Storm arrival time:
Note that brief, gusty showers are possible before the mainline of showers and storms late this evening.
All clear: Midnight west of the Beltway, around 1 a.m. around the Beltway, and 2 a.m. near the Chesapeake Bay.
Rainfall potential: Average 0.25 to 0.5 inches; locally amounts up to 1 inch or so possible.
A look at the day’s forecast map reveals that a warm front pushed through our region last night, ushering in wind from the south and a milder and humid air mass. This front is connected to a deepening low pressure system north of the Great Lakes. Our region will remain in this storm’s warm sector throughout the day and evening, as a cold front approaches from the Ohio Valley.
In the upper levels, a potent trough of low pressure is approaching the Eastern Seaboard. The uplift of air is expected to intensify across the D.C. area as the trough amplifies. With the influx of southern moisture and rising air, waves of showers will develop and transit our region through the afternoon and evening.
Our concern shifts to the early evening, as the front nears and a pocket of unstable air develops just along it. The early morning weather balloon at Dulles Airport shows that the atmosphere — for the moment — is quite stable and not conducive to deep thunderstorms. However, with arrival of milder air from the south (and cooling of the mid levels by the approaching trough), some measure of instability is expected to develop, at least through the middle atmosphere, by the early evening.
Extensive cloud cover through the day will probably prevent the strong, late March sun from destabilizing the atmosphere to any large degree.
While the instability may be somewhat lacking through a deep layer, the low-level wind fields are exceptionally strong and ideally configured such that any deeper cells may achieve rotation. A look at these very strong winds about a mile above the surface is shown below; note that they are in the range of 70+ mph.
Even with shallower convective clouds (lacking significant lightning and thunder), we are concerned that downdrafts within the clouds may bring down blasts of strong wind in locally damaging gusts. The strong wind shear (change in speed and direction with altitude) may also promote transient, rotating storm cells with the possibility of a short-lived tornado or two.
The two images below present radar snapshots as simulated by various forecast models. In the first, the high resolution NAM model suggests a squall line, with embedded strong to severe cells, will pass through the area between 10 p.m. and 1 a.m.
The Capital Weather Gang will stay on top of this scenario through the day and post updates if a severe thunderstorm watch is issued and any local warnings are generated.


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