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11:35 p.m. — Flash flood warning continues as heavy rain exits from west to east

Rain has let up quite a bit west of the Beltway and will continue to decrease from west to east during the next hour or two. Lighter showers may linger though, and the flash flood warning shown below remains in effect until 1:45 a.m.

Flooding and lightning ended up being the main concerns around D.C., with most spots receiving at least one to two inches of rain, and some locally higher amounts up to around three inches. There were also some some reports of large hail. Meanwhile the Baltimore area saw a number reports of wind damage and trees down. All in all an active night across the region.

We’ll update here if any new warnings are issued before midnight. Otherwise see P.M. Update for Ian Livingston’s forecast through tomorrow.

10:55 p.m. — Flash flood warning for much of the DMV as heavy rain continues; Flash Flood Watch until 2 a.m.

Very heavy rain, at a rate of one to two inches in an hour, has prompted the National Weather Service to issue a flash flood warning for D.C., the Beltway and surrounding areas until 1:45 a.m. Life-threatening flash flooding of creeks and streams is possible. This includes urban areas, highways, streets and underpasses. Note that a flash flood watch in effect for the much of the immediate D.C. area until 2 a.m.

10:35 p.m. — Severe thunderstorm warning for D.C. and just north and east; dangerous lightning

A severe thunderstorm warning is in effect until 11:15 p.m. much of D.C., southeast Montgomery County, northern Prince George’s County and into northwestern Anne Arundel County. Lightning has really increased as these storms head east. As always, when thunder roars stay indoors.

10:15 p.m. — Additional warning added in western suburbs as storms close in on on Beltway and D.C.

A new severe thunderstorm warning is in effect until 11 p.m. for areas west of D.C. including Centreville, Oakton, Burke, Annandale and Springfield. All of the activity is heading east, now moving inside the Beltway and the District during the next 30 minutes or so.

10 p.m. — Severe thunderstorm warning mainly west of Beltway until 10:45 p.m. including Ashburn, Reston and Potomac

A severe storm is impacting D.C.'s immediate western suburbs with very heavy rain and potentially damaging wind gusts along with hail and lightning. The storm is tracking east through southeast Loudoun County into northern Fairfax and southern Montgomery counties. An earlier storm produced 1.25-inch hail in Centreville, Va.

To the north, severe storms are producing heavy rain and potentially damaging wind gusts in Columbia and Ellicott City, while Baltimore and surroundings are under a Flash Flood Warning. To the south, an earlier storm produced 1.25-inch hail in Centreville, Va.

9:25 p.m. — Severe thunderstorm warning in central Montgomery and western Howard counties until 10 p.m.

Scattered storms are moving through the D.C. and Baltimore areas from west to east. A particularly strong storm has prompted a severe thunderstorm warning for central Montgomery County and western Howard County, including Clarksburg, Germantown, Gaithersburg, Laytonsville, Brookeville and Olney. Damaging wind gusts near 60 mph and quarter-size hail are possible. Also seeing some lightning.

8:30 p.m. — Severe thunderstorm warning in western suburbs includes central Fairfax and northern Prince William counties

The warning is in effect until 9:15 p.m. and includes Centreville, Reston, Herndon, Fairfax, Vienna, Dulles Airport, Oakton, Chantilly, and Wolf Trap. Damaging wind gusts around 60 mph are possible along with quarter-size hail and dangerous lightning. The storm is tracking east near and north of I-66.

7:35 p.m. — Severe thunderstorm watch issued for D.C.-Baltimore region until 2 a.m. Tuesday.

The National Weather Service has issued a severe thunderstorm watch for the entire D.C.-Baltimore region. Best chance of storms still looks to be around or after 9 p.m. The watch mentions the possibility of scattered damaging wind gusts up 70 mph, isolated hail up to ping pong ball size, and the chance of a couple of tornadoes,

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.