Storm moves away from D.C., flooding and wind damage in its wake

8:15 p.m. — Wrapping up in the D.C. region

Thunderstorms are moving east out of St. Mary’s, Calvert and Ann Arundel and they should be over by 9 p.m. The storm left behind some bad wind damage in northern Virginia — you can scroll down for more of that.

Check out P.M. Update for the forecast through tomorrow. There is a chance we’re going to do this again tomorrow, although hopefully it will be less widespread. In either case, be ready for late afternoon and evening thunderstorms.

7:51 p.m. — Speaking of that wind damage…

Looks like we have some pretty extensive wind damage in at least the Reston area, but it might be more widespread than that. We just got a photo of a pretty terrible tree fall in Reston. This tree was unrooted by strong winds.

One reader told us, “we were walking home from the movie an hour after it had ended. The sidewalk was wet. And then we saw that there were trees down and everything in our neighborhood was dark. And there we saw siding and roofing shingles in the street.”

7:37 p.m. — Possible derecho, just not in the Beltway

The Beltway got lucky tonight. This squall line looked like it had every intention of slamming at least the southern half of the Beltway with the most damaging winds, but it was spared when the storm’s energy focused south of Washington.

We wouldn’t be surprised to see this “bow echo” — a storm that looks like a bow and has very strong winds — actually meet derecho criteria when we sit down to analyze it tomorrow. But it only skirted the Beltway as it consolidated to the south.

Even so, we have wind damage reports in Loudoun and Fairfax counties — trees down and tens of thousands of power outages. There’s also a torrent of water flowing through the District and western suburbs this evening.

7:15 p.m. — An interesting account from POTUS motorcade

The storm most likely did not meet the wind speeds and duration for a derecho, but Steve’s account is interesting nonetheless. The President was at Walter Reed visiting his wife after surgery.

7:05 p.m. — New severe t-storm warning extends into Rockville

The storm continues to push east across the D.C. metro with torrential rain and damaging wind. A new severe thunderstorm warning includes the north and northeast suburbs like Rockville, Aspen Hill, Wheaton/Silver Spring, College Park and Landover.

A flash flood warning is still in effect for the District and near west suburbs, too. A recent storm report included “trash cans floating down street and water over curb” in North Arlington.

6:59 p.m. — Flash flood warning for the District

A flash flood warning is in effect for all of the District as well as the near east suburbs. Don’t walk or drive through flood water. This warning expires at 10 p.m.

6:47 p.m. — Storms intensified rapidly after crossing the mountains

Our severe weather expert Jeff Halverson noted that the leading edge of this squall line really intensified once the system got on the east side of mountains, where it was able to tap into very warm and humid air. This is also how the derecho in 2012 intensified.

We’re showing this in two satellite images, before and after. The darker red areas show where the most intense thunderstorms are located.

Over the mountains:

East side of mountains:

6:35 p.m. — The leading edge of the storm has reached the Beltway

Everyone in the Beltway should be getting inside now, not only because it’s about to pour down rain but because this storm has extremely strong wind gusts that could bring down trees and power lines.

A few reports of damage have come in already — trees down in Loudoun County near Leesburg and there was one report of a funnel cloud in Ashburn, and that was definitely in the tornado warning.

6:22 p.m. — This storm has a massive shelf cloud

We’re getting lots of photos of the shelf cloud from our northwest suburbs. (And while we appreciate the reports, as soon as the storm starts please get inside and find a place away from windows. This storm will bring down trees, branches and power lines.)

6:12 p.m. — Tornado warning for Reston and surrounding suburbs

A new tornado warning is up for parts of Loudoun and Fairfax counties, including Sterling, Reston, Oakton and Great Falls. If you’re in one of these areas, please take shelter in a basement or interior room without windows. Even if there is no tornado, this part of the storm has dangerously strong winds that could bring down trees, branches and power lines.

6:05 p.m. — Strongest part of storm tracking toward Beltway

A tornado warning remains in effect for parts of Loudoun and Montgomery counties, including Leesburg and Poolesville, Md., until 6:15 p.m. The most dangerous part of this squall line is moving southeast toward the Beltway and looks like it will arrive around 6:45 p.m.

A severe thunderstorm warning is now in effect for a large portion of the D.C. metro. No matter where you are, you’re going to get hit by a piece of this storm.

5:53 p.m. — Tornado warning for parts of Loudoun, Montgomery counties

Radar is indicating that a tornado may be present in the strongest part of this squall line in Loudoun County. A tornado warning has been issued for the storm, which includes Purcellville, Leesburg, Ashburn, Poolesville and Sterling.

If you live in these areas, seek shelter in a basement or an interior room. Stay away from windows.

5:35 p.m. — New warnings include far west suburbs

The latest batch of warnings includes most of Loudoun as well as northern Fauquier and northern Prince William. The strongest storm is on the north side of this line, about to roll through the Leesburg area. We wouldn’t be surprised to see some quarter-size hail in this along with torrential rain and gusty winds.

5:20 p.m. — First severe t-storm warning

Expect very large warnings this evening that cover a lot of real estate. That’s the kind of severe weather scenario that will play out over the next few hours. The storms have reached I-81 and are moving southeast at around 30 mph.

Not that we think there will be widespread power outages, but keep your phone charged just in case.

5:00 p.m. update

A severe line of storms has coalesced over eastern West Virginia and continues on a path toward the D.C. area, arriving between 6 and 8 p.m. from west to east. These storms will sweep through Martinsburg, Winchester, and Strasburg along the Interstate 81 corridor over the next hour.

Some key messages about these storms:

• We see the highest risk for widespread damaging winds from the District to the south and west, but everyone should monitor warnings, and stay weather aware this evening. Damaging winds could realistically occur anywhere.
• Charges devices and secure or move inside loose outdoor items now.
• In addition to damaging winds, torrential rain and dangerous lightning are likely in storms.
• A few storms could contain large hail and even an isolated tornado can’t be ruled out
• It’s not a matter of if we see strong to severe storms, it’s a matter of how widespread and damaging they will be. Stay safe and err on the side of caution this evening. If a warning is issued, get inside a strong building, away from windows.

Our next update will be out around 5:30 p.m.

4:00 p.m. update

Vigorous storms in southwest Pennsylvania continue pressing southeast and have now reached western Maryland and extend back through northern West Virginia. They are expected to reach the immediate metro region between 6 and 8 p.m. from west to east, possibly as late as 8 to 9 p.m. in our eastern areas near the Chesapeake Bay.

“The risk for damaging winds is expected to continue, and become potentially widespread, through this evening,” the National Weather Service said in a special statement issued just before 3:30 p.m.

The statement noted that in addition to the wind threat, that the stronger storms may also produce large hail and perhaps a short-lived tornado.

The biggest question at 4 p.m. is not whether the area will see strong to severe storms, but how extensive and damaging they will be.

Per our update at 1:30 p.m., this line of storms could meet the criteria of a derecho, which is a fast-moving line of storms with widespread damaging winds. It is very unlikely, however, this event will be as intense as the destructive June 2012 derecho that swept through the region. We think areas from the District and to the south and west are most likely to see the highest concentration of strong winds.

Now would not be a bad time to prepare for possible power outages. Charge your devices and move inside or secure loose outdoor objects.

Our next update will be around 5 p.m.

2:45 p.m. update

The line of storms which we are concerned about is apparent stretching from southwest Pennsylania to eastern Ohio. It is expected race east-southeast in the coming hours and the arrival time in the metro region is still expected to be between 6 and 8 p.m., from west to east.

The National Weather Service is forecasting a 45 percent chance of damaging winds within 25 miles of any one place, which is an unusually high likelihood for this area. The question is how intense this line of storms will be once it arrives and where it will be strongest. We continue to favor areas from the District and to the south and west for the most intense activity, but everyone should continue to monitor this potentially dangerous line of storms.

Our next update will be around 4 p.m.

1:30p.m. update

The National Weather Service Storm Prediction Center is calling for a potential “derecho” in the Washington region this evening. A derecho – which in Spanish translates to “straight ahead” – is a fast-moving line of thunderstorms with widespread damaging winds.

Many Washingtonians will recall the derecho of June 29, 2012, which unleashed destructive winds of 60 to 80 mph throughout the region, leading to over a million power outages. Not all derechos are created equal and the 2012 case was at the high end of the spectrum. While atmospheric conditions appear to support the development of a derecho, it’s difficult to predict the intensity and whether one will actually form. It is a maybe, not a for sure.

But the fact the Weather Service is discussing the possibility signifies folks should pay very close attention to the weather this evening. If storms warnings are issued and this potentially violent kind of storm complex materializes, you should avoid being outside or commuting until it passes (they move through fast). The safest place to be would be inside a strong building, at the lowest floor, away from windows.

Original post from 12:40 p.m.

For the fourth time in five days, severe thunderstorms could rock parts of the region late this afternoon into the evening. Storms that develop and track through the region have the potential to unleash multiple hazards including torrential rain, dangerous lightning, damaging winds and hail.

For today’s threat, we think the District and areas south have the greatest chance of severe weather,  but everyone, including Washington’s northern suburbs, should stay alert.

In the severe thunderstorm watch issued for the area through 9 p.m., the National Weather Service is calling for the potential for “widespread damaging winds and isolated significant gusts to 75 mph.” It also says some large hail to the size of golf balls are possible and even perhaps a tornado or two.

Of the multiple storm threats we’ve seen over the last several days, this is the most significant as it is possible a solid line of storms containing intense winds, called a squall line, could sweep through the region.

Storm dashboard

• Approximate window when storms are most likely:
  • 4 to 6 p.m.: Interstate 81
  • 5 to 7 p.m.: Washington’s western suburbs
  • 6 to 8 p.m.: Interstate 95 and the Beltway, including the District
  • 7 to 9 p.m.: Washington’s eastern suburbs
• Storm duration: 30 to 60 minutes in any one location
• Chance of measurable rainfall in any location: 60 percent
• Storm motion: Northwest to southeast
• Likely storm effects: Downpours, lightning, strong winds
• Possible storm effects: Damaging winds, small to medium-size hail
• Very small chance of: Large hail, isolated tornado
• Rainfall potential: Averaging 0.5 to 0.75 inches, but locally higher/lower amounts likely

Discussion

The National Weather Service Storm Prediction Center has raised the potential for severe storms, over all of the Washington region, from slight to enhanced this afternoon and evening. The main threat is damaging, straight-line wind (30 percent chance), possibly from a squall line moving out of Ohio/West Virginia.

A corridor for such a system has become established by a stationary front, draped from west to east across Pennsylvania-Maryland. Air on the south side of this boundary is expected to become quite unstable, and wind shear, which helps sustain storms, will increase parallel to this boundary.

As of Monday morning and midday, the region was socked by low-level cloud and pockets of fog. This is because most of the region remains on the cool side of the front, where the air is close to saturation. Weather models suggest that the frontal boundary will move northward through the afternoon, placing the bulk of our immediate region in warmer, unstable air, along with dissipation of low-level clouds.

Much of today’s severe weather forecast hinges on continued northward migration of that front.

The amount of warming will weigh heavily into how much the air mass destabilizes, which in turn determines the highest severe weather potential. At times, history in our region has shown that these fronts may in fact vacillate north-south, and their exact location can be very difficult to pinpoint even with the best model guidance.

Given some uncertainties in this setup, what can we expect for severe weather? Here again we turn to the high resolution model guidance, which is not unanimous. The NAM model only portrays hit-or-miss storm cells during late afternoon and evening, lacking cohesive organization. On the other hand, the HRRR model (shown below) and an experimental, very high resolution model called the HREF (run by the National Severe Storms Laboratory) suggest an organized and intense squall line may sweep through the immediate metro in the 6-8 p.m. time frame.

There is a certain alignment to the front, wind shear, and instability corridor that may indeed favor a bowing, fast-moving storm complex — as suggested by the HRRR and HREF. This does not necessarily mean a derecho, which depends on the storm intensity along its entire track. Bowing squall lines develop all the time and move through our region, with wind gusts and damage that do not reach derecho-level.

If such a storm complex were to evolve later today, the gust and damage potential will be greatest where the atmosphere is most unstable, south and immediately along the weather front. Portions of the storm riding north of the boundary can still generate thundery weather, with heavy rain, but much weaker wind gusts.

The greatest probability for any widespread wind event is portions of Northern Virginia, including Loudon, Prince William, southern Prince Georges and Fairfax counties, and the District. This is where we feel the best chance for sustained warming will occur. All counties along the District’s northern tier stand a lower chance of severe storms, especially if the front moves northward more sluggishly.

Today’s severe weather prediction is highly conditional and there may be a sharp cutoff in our region, in terms of who ends up with the most intense storms.

Another severe storm threat Tuesday

Tuesday, we have to contend again with this troublesome frontal boundary. We expect that the front will have retreated completely north of our region, placing the entire area in a hot, juicy air mass. Once again, we will be ripe for strong to severe storms. The Storm Prediction Center has placed our region square in the bull’s eye of slight severe potential. Of course, stay tuned for more on that Tuesday.