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Intense storms expected in area south of D.C. metro area Friday afternoon

Scattered storms could produce torrential downpours, damaging winds and, perhaps, a brief tornado; a severe thunderstorm watch is in effect

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Radar courtesy MyRadar | © OpenStreetMap contributors

2 p.m. — Severe thunderstorm watch posted for north central Virginia and Southern Maryland

The National Weather Service has issued a severe thunderstorm watch for areas south of the immediate D.C. metro area, as that’s where fuel for severe thunderstorms is most prevalent. To the north, cloud cover has slowed the build-up of instability.

A line of nasty storms has developed along the Interstate 95 corridor between Fredericksburg and Richmond and will sweep eastward through Virginia’s Northern Neck and the southern part of Southern Maryland over the next couple of hours.

In the immediate area, we still expect scattered showers and storms later this afternoon into the evening. Activity currently developing near and west of Interstate 81 is pushing eastward. We can’t rule out isolated severe weather and will monitor the situation.

Original article from 1:30 p.m.

As a cold front approaches the D.C. area this afternoon, it will encounter warm and muggy air, triggering scattered showers and thunderstorms across parts of the area this afternoon and evening.

The main window of storminess locally seems likely to fall between 2 and 7 p.m., moving west to east. In the immediate D.C. area — near the Beltway — timing may favor the 4 to 5 p.m. zone, give or take.

A couple more showers and storms, likely of lesser intensity, may occur into the night before the cold front passes.

Drenching downpours, plentiful lightning and some zones of damaging wind gusts are the main threats from any thunderstorms. An isolated tornado may also develop. As such, the National Weather Service Storm Prediction Center has placed the immediate D.C. area and places south or east under a Level 2 of 5 slight risk for severe storms.

A severe thunderstorm watch will probably be issued for parts of the area this afternoon.

A wild and wonderful Thursday evening sky, followed by a fireball

Discussion

A muggy air mass is entrenched across the area as a cold front approaches from the Ohio River Valley. While the front pushes east, it will help showers and thunderstorms develop out ahead of it.

Widespread cloudiness this morning has limited temperature-induced instability — the fuel for thunderstorms — somewhat thus far. Higher instability is building to the south of the area and some of that may move north as skies clear a bit this afternoon. How far north the unstable air mass makes it is critical for where the strongest storm activity develops and sweeps through.

Relatively strong winds in a river of air aloft, known as the jet stream, plus the summertime instability below should conspire to allow some storms to become intense. At this point, it seems the biggest source of fuel for storms will be located from near the District and to the south, where the Weather Service said a severe thunderstorm watch is likely to be issued.

Some bow-shaped thunderstorms segments capable of strong and potentially damaging winds up to around 60 mph are possible. A rotating storm or two, known as supercells, may also develop given the stronger winds aloft. While the tornado threat is low, a brief touch down can’t be ruled out in any rotating storm. A smattering of hail is also possible, although it should not be too widespread or large.

A lack of storm inhibition — sometimes called a “cap” — noted in Storm Prediction Center discussion could allow showers and storms to develop pretty much whenever, although they’ll probably be most numerous late afternoon

The most widespread risks from any showers and thunderstorms are heavy rain and lightning. With a very humid air mass in place, torrential downpours are possible.

“The potential for multiple rounds of thunderstorms with very high rainfall rates in an air mass characterized by [precipitable water values] over 2 inches could result in an isolated flood threat through this evening,” wrote the Weather Service office serving our region.

These kinds of moisture values can lead to exceptional rainfall rates of one to three inches per hour. This would lead to some flooding potential, especially in urban areas and particularly if storms repeatedly move over the same spots.

Although storms are anticipated to be rather progressive and hit-or-miss, thus limiting the flood threat, some spots remain waterlogged from frequent rains this month. Through Thursday, the District had picked up 7.11 inches of rain in July, compared to the average of 4.2. It’s the 17th wettest July on record to date.

While the main round of showers and storms likely winds down and moves away before sunset, some more showers or even a storm remain possible into the overnight before the front fully passes.

By Saturday morning, considerably drier air will be moving into the area, setting up a beautiful start to the weekend.

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