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Posted at 11:08 AM ET, 07/24/2012

Threat of severe thunderstorms today, highest risk south of D.C.

UPDATE, 12:30 p.m.: NOAA’s Storm Prediction Center indicates a severe thunderstorm watch is likely to be issued for the D.C. area. Already, a large supercell (rotating) thunderstorm has developed in western Maryland in eastern West Virginia for which a tornado warning was issued.

Forecast position of cold front as of 8 p.m.. tonight (National Weather Service)
Original post, from 11:10 a.m.: When a cold front collides with a hot, juicy airmass over the Southeast and mid-Atlantic, strong to severe thunderstorms are possible today. There is the chance of a large, organized area of storms with damaging winds, though it *probably* targets areas south of Washington D.C. from central Virginia into the Carolinas.

Nevertheless, scattered - potentially severe - storms are a risk in the immediate D.C. metro region this afternoon into early this evening.

One area of weakening showers and thunderstorms from southern Pennsylvania may move through the region before that in the 11 am to 1 p.m. time frame around Washington, D.C., but is not our focus at this time

The setup

Upper level flow pattern as of 10 a.m. EDT (Storm Prediction Center, adapted by CWG)
In some ways, the overall configuration of weather systems resembles that of June 29, when the derecho struck the mid-Atlantic and regions to the northwest back towards Chicago.

The jet stream, the region of strong upper level winds, runs from northwest to southeast over top a large heat dome centered east of the Plains. A strong disturbance is tracking along the jet stream. Already today, this disturbance triggered severe thunderstorms around Chicago and more than 180,000 power outages. A wind gust of 58 was recorded just after 6 a.m. local time at O’Hare International airport and some gusts to 60-70 mph were logged in the region.

Late this morning, regional radar showed a large area of strong to severe thunderstorms from northwest Indiana (where the Storm Prediction Center has already logged over 20 wind damage reports) curling into western Ohio and north central Kentucky. These storms will ride along the jet stream towards the mid-Atlantic and Southeast this afternoon.

(Note: despite pronouncements from AccuWeather and The Weather Channel that this convective system is a derecho, it not yet clear it has met derecho criteria. I don’t see a clear 240 mile, uni-directional swath of damage)

The forecast

Chance of damaging winds within 25 miles of given point in shades areas (NOAA Storm Prediction Center)
The Storm Prediction Center has the D.C. area on the northern edge of a large region to the south highlighted for the possibility of severe thunderstorms with damaging winds. Compared to the June 29 event, steering currents are more sharply pointed towards the southeast, so the region from central Virginia into the Carolinas is probably most threatened by this developing storm complex, which *could* evolve into a derecho-type system.

Forecast CAPE values this afternoon. Red areas are above 3,000. (
The storms could lose some punch crossing the mountains, but like June 29, the complex of storms will run into very warm, moist and unstable air on the downwind side. Levels of Convective Available Potential Energy (CAPE), a measure of instability, are forecast to be very high, in the 3,000-4,000 range - though somewhat less than June 29 (over 5,000).

Even if the D.C. area is north of an organized complex of severe weather (appears the most likely scenario), scattered areas of strong to severe thunderstorms are still possible.

Storms that develop could contain damaging winds and/or hail. The most likely timing for storms is between 3 and 8 p.m.

To conclude, while the risk of widespread severe storms is highest south and southwest of the metro region and lowest to the northeast - the entire region should be on guard for the potential for strong to severe storms this afternoon and evening.

By  |  11:08 AM ET, 07/24/2012

Categories:  Latest, Thunderstorms

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