6:10 p.m. update: There is currently no severe weather in the D.C. metro region and the threat of severe weather has greatly diminished. Officially, a severe thunderstorm watch remains in effect until 8 p.m. but may be discontinued a bit early. Obviously, if any storms of significance pop-up in the next couple hours, we’ll update this post. If not, this is the last update. Here’s the forecast through Wednesday:
Rest of tonight: After any storms pass to the south, expect gradual clearing and drier air to filter in. Lows range from near 60 in the cooler suburbs to near 70 downtown.
Wednesday: Sunny and really quite nice. Probably the nicest day this week. Highs reach the mid-to-upper 80s but humidity is low-to-moderate, with dew points in the 50s.
It gets really hot Thursday. See Matt Rogers’ forecast through the weekend.
6:00 p.m. update: Storm in Montgomery county has weakened and is heading towards northern Prince George’s county if it doesn’t fizzle out entirely. The District, and Fairfax and Loudoun counties may well miss out on storms entirely.
5:35 p.m. update: Storm in Montgomery county along I-270 has strengthened a bit prompting a severe thunderstorm warning until 6 p.m. Germantown, Gaithersburg, Rockville, North Potomac and Olney are in the potential path. This is just a borderline severe storm; the NWS seems to be erring on the side of caution here.
Coverage: The most concentrated area of severe weather is likely in the southern half of the watch box. Around Washington, D.C. and Baltimore, any thunderstorm activity will likely be more isolated to scattered (i.e. hit or miss).
Impacts: Storms that form are likely to contain heavy rain and dangerous lightning. Large hail (to 2”) and damaging winds to 75 mph are possible in storms that develop. In addition, an isolated tornado cannot be ruled out.
This is not a derecho event for the D.C. area (though locations in southwest Virginia into North Carolina could experience one), but a few large thunderstorms with destructive winds are possible.
Timing: Severe storms are already affecting the mountains (e.g. Shenandoah county). In the D.C. metro region, the most likely timing for storms is probably between 3 and 7 p.m., but adjustments may be required.
Cause: The trigger for these storms is a cold front to the north sliding south into our hot, unstable air mass and an active jet stream.
5:05 p.m. update: The storm in Frederick county, passing through Frederick now, has weakened some and is no longer severe. It’s headed south into northern Montgomery and Howard counties, but it remains to be seen what will be left of it by the time it draws closer to the immediate D.C. area between 5:30 and 6:30 p.m.
Want to see some real storms? Check out the radar view from eastern North Carolina, where a dangerous derecho-like squall line is battering the region.
4:30 p.m. update:The storm moving through Frederick county has turned severe (warning until 5 p.m.) and is the storm to watch for the immediate D.C. area. It should enter Montgomery county around 5 p.m. and could affect areas inside the beltway between 5:30 and 6:30, including the District, if it holds together.
4:15 p.m. update: Storm activity remains pretty isolated and may stay that way. The one cell near Winchester is sliding more to the south than to the east. So while it will hit areas like Front Royal and probably Rappahannock and Fauquier counties, it may not make it into D.C.’s closer-in southern suburbs.
The one storm to watch is in northern Frederick county which may reach northern Montgomery and Howard county in the next hour.
Neither of these storms is severe, but either/both could intensify a bit.
3:45 p.m. update: An area of strong storms out towards Winchester (in Frederick co.) bears watching for D.C.’s southern suburbs (southern Loudoun, northern Fauquier, northern Prince William and perhaps southern Fairfax co) in the 5 to 6 p.m. timeframe. There’s also a strong thunderstorm just north of the Maryland border that may affect Frederick and Carroll counties in the next hour.
3:00 p.m. update:Similar to yesterday, the D.C. area is in a hole between areas of thunderstorms to the north and south. However, high resolution models show isolated to scattered activity - originating from southwest Pennsylvania - affecting the area mainly in the 4 to 7 p.m. window. Storms may well be hit or miss, but any that form could become severe given dynamics in place. Chance of storms is in the 30-40 percent range with the highest odds south of the District.
2:10 p.m. update: It’s relatively quiet in the D.C. area for now. Even out towards the mountains to the southwest, where there were severe thunderstorm and tornado warnings last hour, the storms have weakened.
The area for the metro region to watch is to the northwest. Numerous showers and storms are developing in southwest Pa. This activity could reach the metro region later this afternoon and/or new storms could develop out in front of it.
The really nasty storms are in northeast Kentucky and southwest West Virginia. You *might* be able to call this bowing line of storms a small-scale derecho. Wind damage reports are mounting. It’s headed towards southwest Virginia and perhaps into the Carolinas.