Derecho Hits the Mid Atlantic Region

A massive storm complex roared into the Mid Atlantic Region on Friday, June 29, resulting in damage similar to the effects from a tropical storm or hurricane. Destructive winds of around 70 mph were recorded as the storm moved through the area.

This particular type of storm is known as a “derecho” and is most often seen in the Midwest. Derecho refers the “straight ahead” motion and wind pattern of the storm (as opposed to the rotation of a tornado) as well as its’ fast forward motion and long life.

This particular storm was unusually large. The satellite image above is from the early evening and shows the gust front moving into West Virginia.

The radar image above shows the rain pattern at roughly the same time as the previous satellite image. The enhanced red area is the intense rain along the gust front at the leading edge of the storm.

The derecho took shape in Iowa early Friday morning as a collection of thunderstorms that organized into a “mesoscale convective system”. After several hours duration and strengthening the MCS escalated to a “derecho’ and traveled all the way to the Atlantic Coast by the pre dawn hours of Saturday.

Source: NOAA

The image above is a composite of radar images from the entire day on Friday and the path of the storm is obvious.

Between 10:00pm and Midnight the storm moved into the Baltimore-Washington area, resulting in considerable damage, especially to trees and power lines.

The radar image above shows the extent of the rain just before 11:00pm with the squall line reaching from southern Pennsylvania to northern North Carolina.

This storm was unusual in several respects. First, it managed to cross the mountains intact. It was extremely strong going into the mountains and ran into a very favorable environment east of the ridges with an unstable airmass and abundant moisture. Second, its’ size. Third, its’ speed. The storm had a forward speed of roughly 60 mph for much of it’s life, moving from Chicago to Baltimore in about 12 hours. Fourth, its’ long life. Derechos typically are long lived storms and this storm was at the outer limits of what is typical.

The image above is a diagram of the structure of a derecho. It is part of a NOAA article on derechos that is available on the web at

The destruction caused by the storm was enhanced by the fact that the Mid Atlantic Region is in the middle of a heat wave. Hundreds of thousands of power customers across the region were left without lights, air conditioning and other necessities. Because of the short fuse nature of the storm, utility companies were given little time to prepare for the storm and have been hard pressed to restore service to customers quickly.

John Collins

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