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Capital Weather Gang

D.C. endures worst storm since 2012 derecho — here’s how it happened

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July 1, 2015 at 10:59 a.m. EDT

In the middle of the night, in July’s opening moments, the most violent complex of the storms since the June 2012 derecho blasted the immediate D.C. area. It downed scores of trees and produced blinding rain and almost non-stop lightning as it swept straight up the I-95 corridor from near Dale City through the District and into Baltimore.

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This morning, area utilities, including Pepco, are still dealing with thousands without electricity.

Many eyewitnesses, awakened and rattled by the storm, described it as the scariest they could recall — jarred by the deafening thunder, continuous lightning, pounding rain and roaring winds.

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The intense but very localized storm complex formed from the merging of multiple thunderstorm cells just to the south of Washington after midnight.

Cell mergers are known to produce exceptionally strong thunderstorms. The updraft and rain mass of one cell combines with similar features of another cell. The combination can lead to a surge in updraft intensity, lightning generation and subsequent generation of damaging winds.

Two radar snapshots illustrate the cell merger process, shown below.

The storms were enhanced by a dynamic region of high altitude spin approaching from the west called a vorticity maximum. The vorticity was part of a ripple in the jet stream, called a shortwave trough, which had moved steadily across the Ohio Valley. Earlier, on June 30, this impulse triggered an expanding arc of strong to locally severe thunderstorms across Ohio and West Virginia.

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The levels of two key atmospheric ingredients normally required for severe thunderstorms, instability and wind shear (the increase in wind speed with altitude), were only modest.

This was a small scale event, not a derecho by any means, but one that illustrates how seemingly innocuous ingredients in their own right can create quite a synergy once blended.

A derecho is a much larger and faster moving weather system, characterized by a swath of damaging winds at least 240 miles long.

The wind

The wind damage observed across the area occurred from phenomena known as wet microbursts — in which powerful high altitude winds blast down to the ground and spread out in all directions. One or more microbursts were likely responsible for the wind damage in the region.

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The core of strongest winds tracked through Alexandria, the District and Prince George’s County, where we received the most reports of downed trees.

The highest wind gusts were logged in central Prince George’s County, just east of the District. Seat Pleasant, Md., recorded an astonishing gust to 74 mph, while Mt. Rainier clocked a gust to 66 mph. Reagan National Airport recorded a peak wind gust of 51 mph just after midnight.

In Bowie, Md., the violent winds flipped over a canopy at a gas station. No one was hurt.

Some images of downed trees around the region:

The lightning

Residents described lightning as near-continuous, to the point of being frightening.

Watch this dramatic animation of lightning strike density:

Look closely and you will see several cells approaching southeastern Fairfax County from different directions — an impulse from the southwest converging with cells arriving from the south. The merged complex then blows up across the southern tier of the District, moving off to the northeast.

In short, the merging thunderstorms and the vast amount of energy released in the process spurred the lightning display, best described as psychedelic.

Some pictures and video:

Rainfall amounts

The storms also unloaded torrential rainfall as they swept through, leading to localized flooding.

Reagan National Airport received 0.97 inches of rain in less than an hour. The White House rain gauge reported 1.03 inches from the cloudburst.

The storm put an exclamation point on what has been an exceptionally stormy 31 days, in which more than a foot of rain has fallen along the I-95 corridor from Alexandria to Baltimore.

Social video and photos capture the storm that woke many across the District Tuesday night and the damage that followed Wednesday morning. (Video: Sarah Parnass/The Washington Post)
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