1:20 PM Update | *****Widespread severe wind event on the way for DC/MD/VA and likely extends up across southern PA including the Philly metro region*****

Showers and thunderstorms in the western part of the Mid-Atlantic region will increase in coverage and intensity later today and travel east towards the I-95 corridor. A “swirl” in the radar echoes can be seen over southwestern NY indicative of a small-scale low pressure system that is adding to the upward motion in the Mid-Atlantic region. Images courtesy NOAA, College of DuPage

Follow-up analysis:

Numerous damaging wind reports in the Mid-Atlantic region were reported to NOAA’s Storm Prediction Center following the weather event’s of Monday, August 7th. Map courtesy NOAA/SPC

Heavy downpours accompanied the weather event’s of Monday, August 7th in the Mid-Atlantic region with the DC metro region featuring higher total rainfall amounts than the Philly metro area which, in turn, featured more rainfall than New York City. Map courtesy NOAA/NWS

1:20 PM (Monday) update:

The severe weather threat is increasing across Maryland, Virginia and the Washington, D.C. metro region where skies have cleared allowing for temperatures to climb well up into the 80’s. Clouds should thin out across southern PA during the next couple of hours. This daytime heating is helping to destabilize the atmosphere setting the stage for what appears to be a widespread severe wind event later today into early tonight across Maryland, Virginia and Washington, D.C. and it is likely to extend up across much of southern PA including the Philly metro area. In addition to the low-level heating, ingredients include ample available moisture and powerful jet streaks at multiple levels of the atmosphere (e.g., 250 millibars, 850 millibars). The potential exists for wind gusts to reach into the 60-75 mph range with power outages on the table. Tornadoes are also in the picture during this weather event…expect a tornado watch to be issued shortly for many areas. The likely timetable for the severe weather in the DC-to-Philly corridor is 4-8pm.

Monday morning original posting…

Overview

Severe thunderstorms are a threat from later today into tonight throughout the Mid-Atlantic region with damaging wind gusts the primary risk; especially, in the region extending southward from southern Pennsylvania to DC/Virginia. A vigorous wave in the upper atmosphere will combine with other ingredients such as strong upper-level jet streaks, plenty of low-level moisture and a surface front to enhance upward motion in the Mid-Atlantic region and, in turn, increase the chance for severe thunderstorm activity. While showers and thunderstorms can break out at just about any time, the most likely time period with the greatest chance of severe weather will be about 2-9 PM.

Multiple jet streaks in the atmosphere are adding to the threat of severe weather in the Mid-Atlantic region and damaging wind gusts as a primary risk (850 mb (low-level), left; 250 mb (upper-level), right). Maps courtesy NOAA, tropicaltidbits.com

Details

Mix together an incoming potent upper-level trough with plenty of low-level moisture, a surface front, and some daytime heating and watch out for a quick destabilization of the atmosphere. In fact, radar echoes are already showing increasing coverage this morning across the western Mid-Atlantic with a “swirl” in the pattern seen over the eastern Ohio Valley indicative of a mesoscale low pressure system that is adding fuel to the fire. Showers and thunderstorms will continue to increase in coverage and intensity though mid-day hours as upward motion increases in the Mid-Atlantic region and severe weather will be possible in the DC-to-Philly-to-NYC corridor from early-to-mid afternoon through the late evening hours.

“Bowing” can take place later today in a squall line that is likely to form across southern PA, Maryland, Virginia and Washington, D.C. “Bowing” is often seen in a region with powerful surface winds (and damaging wind gusts) and in this case, 60-70 mph gusts are on the table across Maryland, Virginia and the DC metro region. Map courtesy NOAA, tropicaltidbits.com

Jet streaks at multiple levels of the atmosphere today will enhance shear in the Mid-Atlantic region and damaging wind gusts of 60-70 mph could very well be the result as “supercell” thunderstorms initially form in the increasingly unstable atmosphere and ultimately, evolve into a squall line.  This squall line is likely to develop some “bowing” later this afternoon indicative of some very strong winds at the lower levels of the atmosphere and this leads to the high concern of damaging wind gusts; especially, across southern PA, Maryland, Virginia and the Washington, D.C. metro region. 

Another key ingredient to today’s severe weather threat in the Mid-Atlantic region is a potent wave at the 500 millibar level that is crossing the Ohio Valley and enhancing upward motion out ahead of it in the Mid-Atlantic region. Map courtesy NOAA, tropicaltidbits.com

In addition to the high concern for damaging wind gusts, other severe weather parameters are on the table including hail, torrential rainfall (brief), and even isolated tornadoes. Thunderstorms should wind down late in the evening and clear the east coast by midnight or so and then drier air behind the frontal passage will arrive in the I-95 corridor by later in the day on Tuesday as surface winds become northwesterly.

Severe weather probability is “enhanced” to “moderate” in much of the Mid-Atlantic region according to NOAA’s Storm Prediction Center.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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