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)

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)

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)

Forecast CAPE values this afternoon. Red areas are above 3,000. (Twisterdata.com)

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.

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.

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