4:35 p.m. — Severe thunderstorm watch issued for far northwest part of region, near Interstate 81. Storms may develop in immediate area starting around 9 p.m.
As some storms fire up in southwest Pennsylvania, the National Weather Service has issued a severe thunderstorm watch for northwest Virginia, the eastern panhandle of West Virginia and western Maryland until 9 p.m.
Storms may start to invade this zone between about 7 and 9 p.m.
Closer to the immediate area, we may see storms start to flare up after 9 p.m., on the early side of the window described below. It would not surprise us if a severe thunderstorm watch is issued.
It may be that, instead of a solid line forming and coming through after 11 p.m., we see scattered storms in the region before that, which sap the energy from any line that might come through later. However, these initial scattered storms may be severe, with a risk of very heavy rain, lightning and localized damaging winds. And, as noted below, some hail and an isolated tornado can’t be ruled out.
We’ll post additional updates as storms develop.
Original article from mid-afternoon
A strong cold front sweeping across the Washington region tonight may produce a line of strong to severe storms. The most likely time window for storminess is between about 8 p.m. and 2 a.m., affecting the western part of our region first.
The storms could produce scattered damaging wind gusts in addition to heavy rain and lightning. Some hail and even an isolated tornado are possible.
The National Weather Service Storm Prediction Center has placed the region in a Level 2 (out of 5) threat zone for severe storms.
Some widely scattered storms could pop up ahead of the main line between 7 and 10 p.m.
When the main line of storms comes through, probably after 10 p.m. in the immediate area, the most widespread and intense activity may be focused north of the District.
At a glance
Timing for main line:
- Interstate 81 (Hagerstown/Winchester/Front Royal): 9 to 11 p.m.
- Route 15 (Frederick/Leesburg/Gainesville): 10 p.m. to midnight
- Interstate 95 (Baltimore/Washington/Fredericksburg): 11 p.m. to 1 a.m.
- Route 301 (Crofton/Upper Marlboro/Waldorf): 11:30 p.m. to 1:30 a.m.
Widely scattered storms could develop two to three hours before this main line passes through.
Storm motion and duration: Moving northwest to southeast and lasting 30 minutes or so.
Main threats: Heavy rain, lightning, localized damaging wind gusts.
Small chance of: Hail, isolated tornado.
Rainfall potential: Highly variable. Up to an inch or so in the heaviest storms.
All clear: By 2 a.m.
Confidence: Medium. It’s uncertain how many storms will develop ahead of the main line and how intense and solid the main line will be. It may break apart, especially from the District southward.
Discussion
It may seem unusual to be concerned about severe storms in our region during the later evening hours up until midnight, but it has happened. The setup for late today is potent enough to give us some level of concern. With most people hunkering down for the night, it will be important to remain alert.
The larger atmospheric setup features a potent wave of energy, or “trough,” in the jet stream ejecting out of the Great Lakes into the Mid-Atlantic, triggering a weak wave of low pressure and frontal passage near the surface.
The trough will increase the intensity of winds aloft, which means wind shear will be on the rise through the afternoon and evening. In fact, a group of model simulations predicts an unseasonably strong peak in shear of around 50 mph this evening. The graphic below shows the various simulations, with the solid dark line presenting the average. The shear maxes out at 8 p.m. (00z in graphic below, referring to universal time.)
A complex of thunderstorm cells is expected to fire during the late afternoon across southern Pennsylvania and Western Maryland. Shear of this magnitude has the potential to organize a squall line into an undulating storm called a “quasi linear convective system” (QLCS). These intense systems have embedded, small circulations and bowing regions where surface winds surge ahead.
Shear of this magnitude can also trigger a supercell (a discrete rotating storm cell) or two.
The big question will be how much unstable air will be present during the evening hours. This is a critical piece to the severe-weather threat. Typically, instability wanes in the hours approaching sunset, with the loss in surface heating. The same group of model simulations that we use to predict shear shows a robust degree of instability remaining in the atmosphere heading into darkness.
Here, there is considerable spread in the simulations, but the solid black line portrays the average. A value of 2000 joules per kilogram of buoyant energy is plenty healthy for strong to severe storms, and that value is approached at 8 p.m. (00z) with values remaining elevated through 10 p.m. Winds from the south ushering in warmth and humid air apparently are helping to offset the loss of solar heating.
It’s entirely possible that the Storm Prediction Center will place our region under a severe-thunderstorm watch later today. A watch has already been issued for areas north of the Mason-Dixon Line.
For now, the Storm Prediction Center feels that the risk of widespread, damaging winds is 15 percent within 25 miles of a point in our area, and the chance of a tornado is around 2 percent.
"severe" - Google News
June 15, 2021 at 06:35AM
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UPDATES: Severe thunderstorm watch issued as storms may strike after dark in D.C. area - The Washington Post
"severe" - Google News
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