‘Widespread damaging winds’ could affect the Washington region late this afternoon and evening

* Severe thunderstorm watch until 9 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
  • 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.