Historic Northeast Derecho, Worst Since ’90s, Strikes D.C.

What a derecho! I couldn’t believe my eyes last night when a derecho of historic proportions crossed the Appalachians and entered northern Virginia, D.C., and Maryland.

The bulging ridge of high pressure that has brought record heat across large chunks of the country also lead to a very favorable setup for significant severe on its northern periphery. In fact, the environment preceding Friday’s derecho was one of the most impressive, and unstable,  you’ll ever see in the northeast.

With a whopping 5686 j/kg of mixed layer CAPE and MUCAPE of nearly 7000 joules here at Dulles Airport the atmosphere was primed for a truly special severe weather event. Mixed layer lifted index at IAD was an exceptional -15c!!!! An elevated mixed layer that advected into the Mid Atlantic around the edge of the huge ridge over the central and southeast U.S. primed the atmosphere for extreme instability.

Convective initiation occurred in the upper midwest over Iowa with a subtle shortwave and warm front/moisture discontinuity early on Friday. As the convection traveled east it encountered greater instability and sufficient shear to develop a substantial mesoscale convective system. The system became a derecho producing a swath of wind damage that rivals some of the great derechos in U.S. history.

Each blue dot represents a wind damage report (or winds greater than 50 knots) while each black square indicates significant severe (winds greater than 65 knots). For east coast derechos this one appears to be one of the most significant in history. The exception may be the July 1995 series of derechos, including the infamous Adirondack derecho, which may have been more impressive as a whole.

The derecho as it was entering the metropolitan D.C. area was about as classic as you’ll ever see on radar.

I labeled the above 0.5 degree LWX base velocity image from 10:05 p.m. last night. The #1 just west of Leesburg, VA shows the damaging wind that was reaching the ground at the leading edge of the derecho. Doppler radar showed radial wind velocities of up to 75 knots as low as 1100 feet AGL!

The #2 labeled above is the rear inflow jet which was helping accelerate the apex of this bow echo forward. The RIJ had descended to the surface resulting in an unusually long and intense periods of damaging winds that is characteristic of a derecho.

The mature bow echo also had a classic rear inflow notch/weak echo channel as it approached D.C. Frequently these are seen with a local maxima in the rear inflow jet as is evident on the base reflectivity near Berryville, VA. In fact velocities here about 3kft AGL were in excess of 85 knots!!

The derecho produced widespread wind gusts near hurricane force around D.C. and power outages similar to what you’d expect following a weakening hurricane or tropical storm. Temperatures approaching 100 and humidity will make for a miserable few days around D.C.


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