Overview: The National Weather Service Storm Prediction Center has issued a severe thunderstorm watch for the D.C. and Baltimore regions, extending into southern New York State, until 9 p.m.

The greater concentration of severe storms is likely to occur in the northern half of the watch area, where several severe thunderstorm warnings are currently in effect. But even here in the D.C. area, storms may produce some damaging wind gusts and hail later this afternoon and evening. We think the most likely timing for storms is between 6-9 p.m., although isolated showers/storms could occur earlier.

Area under severe thunderstorm watch (National Weather Service)
Area under severe thunderstorm watch (National Weather Service)

Remember that a watch means the environment is favorable for development of severe storms but no guarantee. It’s essentially a yellow light: use caution and stay alert. A warning – on other hand – is effectively a red light: it means severe storms are occurring or imminent and you should seek shelter.

5:15 p.m. update: For the latest information on the storms, see this post: PM Update: Scattered storms this evening, possibly severe

4:44 p.m. update: A severe thunderstorm warning has been issued for central Frederick County, extreme northern Montgomery County, northwest Howard County and western Baltimore County until 5:30 p.m.  Storm just north of Frederick moving east-southeast towards Mt. Airy and Eldersburg.  This storm may contain some hail and winds to 60 mph.

4:20 p.m. update: A severe thunderstorm warning has been issued for northern Frederick (Md.) County until 4:45 p.m.  A strong 8 miles northwest of Walkersville is moving east at 20 mph.  It may graze downtown Frederick, but the worst of the cell should pass on its north side.  The storm may contain some hail and damaging winds to around 60 mph.

4:10 p.m. update: With the exception of an isolated storm north of Frederick, Md., it’s pretty quiet out there for now.  But storms are organizing from south central Pennsylvania through western Maryland.  Short-range models suggest a line by develop and will extend far enough south to potentially impact the region between roughly 5 p.m. and 9 p.m. from northwest to southeast.  We’ll keep on eye on this.

3:15 p.m. update: Technical discussion from Jeff Halverson:

With strong heating and advection, CAPE has become significant at 2500 J/kg and lifted is -6. We have 1.6″ precipitable water, which is a significant value. The atmosphere is thus strongly destabilized. What we still lack, however, is strong shear (turning of winds with altitude).

I’m still leaning toward pulse-type severe storms, however, enough shear has developed to promote some organized and stronger storm growth. By organized, I am referring to multi cell-type storms, and possibly a moderate squall line pushing east off the mountains later this afternoon into the evening.

Low-level lapse rates are very large, so there is a potential for isolated wet microbursts. Hail parameters do not suggest the type of large stone growth we experienced last week, but could include dime to quarter size in isolated cells. With the very moist atmosphere and relatively slow storm motion, local flash flooding may be a concern. And as usual, anytime we have large instability, intense lightning activity is always a concern.

(National Weather Service)

From 2:00 p.m.: Keep an eye on radar throughout this afternoon and evening, as numerous showers and thunderstorms are likely to develop.

* Through this afternoon, scattered “pop-up” showers and storms are most likely
* This evening, a more solid line may try to push through the region (see animation below) which could affect the latter half of the evening commute and the beginning of the Nationals game (6-10 p.m. timeframe)

Simulated radar animation of possible line of showers and storms this evening (WeatherBell.com)

While storms are likely, we do not foresee widespread severe weather.

Although convective available potential energy (CAPE) could be moderate given the degree of heating and moisture, we believe we’ll lack the wind shear (turning winds with altitude) for higher levels of storm organization.  Triggers/convergence zones for storm development will be limited to heated terrain, a lee trough (low pressure east of the mountains) and the Bay breeze.  A shortwave zipping through may ignite a line of storms in the 6-10 p.m. timeframe.

The most likely mode for organization will be pulse-type severe – i.e. isolated and short-lived.  General thunderstorm coverage could be fairly widespread, however.  Both the WRF-ARW and HRRR mesoscale  (see above) models are aggressive with developing widespread storms over the high terrain, and slowly advecting this line toward the cities during the evening.

Effects from any intense storms would be limited to isolated strong wind gusts and small hail.

Caveat:  Like we saw last week, any line that does get organized on the lee slopes (downwind side of Appalachians) can create a strong outflow boundary, which in turn can propagate convection further east, in spite of weak shear.  Because of moderate instability, strong storms would be a bit more widespread, especially over the western suburbs, under this scenario.