- Scattered storms possible this evening, through around 10 p.m.
- Storm coverage will be hit or miss, but with a greater concentration west and northwest of the District.
- Isolated storms may be severe, with the primary impacts heavy rain, lightning and, possibly, small hail and damaging winds
- A severe thunderstorm watch is in effect until 10 p.m. for the D.C. metro area and northern Maryland, but excludes suburbs south of Fairfax and Prince George’s Counties.
Update, 5:45 p.m.: Some good news- the line of storms to our north looks to stay to our north mostly and has weakened. It stretches from the Montgomery County/Frederick County line up through western Carroll County. Some downpours and a bit of lightning is likely as the line pushes through northern Montgomery and Howard Counties.
This is the last update in this post. There are some storms in northern Fauquier County moving east which could affect the immediate D.C. area between 6 and 8 p.m., although they are not particularly strong at the moment. Here’s our detailed forecast through tomorrow: PM Update: Storms possible this evening; drying out Wednesday, still hot
Update, 5:17 p.m.: The line to watch now is pushing through Frederick. It spans from northern Loudoun County across the Mason Dixon line. A severe thunderstorm warning is in effect through 6 p.m. for eastern Frederick County, extreme northern Montgomery County, northern Howard County, and much of Carroll County.
Incredible shelf cloud entering Frederick MD. Heads up, buts indoors! Thnx Patrick Smith pic.twitter.com/QNnvLd4PvS— Justin Berk (@JustinWeather) September 2, 2014
Update, 5:10 p.m.: The easternmost storm which tracked across Montgomery County and through Laurel earlier has now moved into northern Anne Arundel County. It has weakened slightly but will produce some downpours, gusty winds, and lightning around Severn, Glen Burnie, and Severna Park.
Update, 5:00 p.m.: The severe thunderstorm watch has been expanded south and now includes the immediate D.C. metro area. It excludes our far southern suburbs, south of Fairfax and Prince George’s County.
Also, a new severe thunderstorm warning has been issued for northern Loudoun, southern Frederick and extreme western Montgomery County. This is part of an intense line of storms moving through northern Maryland which may extend south and clip the D.C. metro area over the next couple of hours.
Update, 4:40 p.m.: A severe thunderstorm warning has been issued for eastern Montgomery, northern Prince George’s and southern Howard County until 5:15 p.m. Storm near Olney is moving east at 30 mph and headed in the direction of Laurel. Torrential downpours and some strong winds are likely.
In addition, a severe thunderstorm watch has been issued for northern Maryland, including Frederick, Baltimore Carroll counties, until 10 p.m. A nasty line of storms is entering Frederick County, and a severe thunderstorm warning is in effect through 5:15 p.m. for the western and central part of the county, including Frederick.
Update, 4:30 p.m.: The Montgomery County storm has pushed east of I-270. Olney and Aspen Hill are up next for this storm, which is producing very heavy rain.
Update, 4:20 p.m.: Much of Montgomery County is under a severe thunderstorm warning through 5 p.m. Gaithersburg and Rockville are about to get hammered. Seek shelter in these areas – damaging winds and small hail possible.
Update, 3:55 p.m.: A severe thunderstorm warning has been issued for eastern Loudoun, western Montgomery, and extreme northern Fairfax counties until 4:30 p.m. A severe storm that had been near Leesburg is moving southeast at 30 mph. Small hail and damaging wind gusts (to 60 mph) are possible in this storm, headed towards Poolesville and Darnestown.
Original post 2:48 p.m.
A weak approaching cold front and the hot, humid air mass is rounding out the usual summer recipe for afternoon-evening thunderstorms on Tuesday. While the highest chance for thunderstorms will be west of the D.C. area, there remains a chance of storms in the District itself later this evening, and scattered showers have already begun to form along I-81 this afternoon.
Overview of today’s thunderstorm risk
The Storm Prediction Center (SPC) foresees a general risk of thunderstorms for the Washington D.C. region, during the late afternoon-evening hours (Figure 1, above). Overall, it’s a slight risk category, oriented along and ahead of the cold front as it moves out of the Ohio Valley. The risk is for localized wind damage, with a 15 percent probability in the core of the yellow zone. Washington D.C. and immediate suburbs carry a 5 percent wind damage risk. According to SPC, the risk for tornadoes and hail today is zero.
Large-scale (synoptic) setup
Figure 2 shows the advancing cold front, valid for 8 p.m. Tuesday evening, which is very weak in terms of temperature contrast. It’s so weak, in fact, that the front will not usher in much of a cool down over the next couple days – only a drop in humidity and shift in the winds.
Ahead of the front, the Bermuda High continues to pump in a plume of stiflingly hot air and high humidity. Dew points are in the low 70s area-wide, and precipitable water (a measure of how much moisture would “rain out” from a vertical air column) is 1.8 inches, a very large value.
The atmosphere is destabilizing, due to intense sunshine and the high humidity. However, a look at the morning Dulles balloon sounding reveals significantly warm air in the upper levels of the atmosphere, which may limit the amount of destabilization (an unstable atmosphere is one that is heated strongly from BELOW, whereas warming aloft tends to stabilize the air column). Still, modest values of convective (buoyant) energy are expected to develop by mid-afternoon.
Given the combination of frontal lift, unstable air, and abundant moisture, the classic three ingredients for thunderstorms are in place.
What may limit storm severity
Without wind shear (winds increasing speed with altitude) thunderstorms can not become long-lived or organized. Wind shear over the D.C. region today is quite weak, around 15 to 20 knots of speed increase in the lowest few miles. And with weak flow aloft, thunderstorm cells will tend to move sluggishly from west to east.
Figure 3 illustrates where the strong shear is located, as of 11:30 Tuesday morning – back across Ohio and western Pennsylvania. The short-term forecast models predict that this pocket of fast wind will remain to our north and west, as the front sags into our region this evening. This is likely the main reason that SPC shows the greatest severe storm threat across West Virginia and west-central Pennsylvania.
What the models say
The high resolution models such as the HRRR (Figure 4) and WRF-ARW (Figure 5) suggest scattered convection will develop over the mountains and sweep across the metro region this evening. The HRRR (valid at 8 p.m. Tuesday evening) really tones down the convective coverage, but even the WRF-ARW indicates a bit of a gap in activity across the central Maryland-Northern Virginia area.
Given the limited instability (due to warm air aloft) and weak shear, I think the model portrayal is reasonable. The most likely mode of convection will involve weakly organized multicells and short lines of showers and storms, moving about 25 mph from the west, from 4 p.m. to 10 p.m. I do not foresee widespread, fast-hitting severe activity; however, storms may form a broken or semi-solid line ahead of the front.
In terms of hazards, a few localized, wet microbursts are possible, with peak wind gusts in the 50 to 60 mph range. Lightning may be frequent, based on the manner in which buoyant energy is distributed in the balloon sounding. Additionally – given high precipitable water and sluggish movement of storms – some localized flash flooding is possible, especially in regions that have gotten drenched over the past two days.
I expect that storm events may play out similar to what we saw on Monday afternoon, in terms of coverage and intensity. Local warnings are possible as a few storms briefly shoot to strong or low-end severe levels.