May 14 squall line on radar. (Radarscope)

Monday night’s vigorous and sometimes violent storms probably did not quite meet the criteria of a derecho, which is a fast-moving, destructive windstorm that leaves behind widespread damage. But it will be recalled as in intense squall line that unleashed heavy rain, strong winds, some hail and pockets of damage around the region.

How it evolved

On Monday, by midmorning, several weather prediction models suggested an organized, bow-shaped complex of thunderstorms would congeal over southeastern Ohio during the midafternoon and race toward the southeast. The storm would ride along a stalled front, drawing heat and moisture building on the south side of the front. Indeed, this is what happened.

By the time the storm crossed the eastern slopes of the Blue Ridge, the squall line morphed into the classic bow shape and a flurry of severe thunderstorm warnings were dispatched for D.C.’s western suburbs. The greatest concern was a surge of damaging, straight line wind along the bowing segment, but additionally, two tornado warnings were issued for Loudoun and Fairfax counties. The radar (above) and satellite presentations of this mature storm system were very impressive.

Through the morning and afternoon, we correctly reasoned the worst corridor of damage would align south of the Potomac, since this is where the greatest concentration of unstable air would reside. Isolated pockets of significant wind damage unfolded across several Northern Virginia counties, and there were several reports of large hail. The graphic below presents a summary of damage and peak recorded wind speeds for the event.


Map of storm damage including wind damage reports, wind observations, funnel cloud and hail. Orange polygons are severe thunderstorm warnings issued by National Weather Service. (Jordan Tessler)

Was this storm a derecho?

On Monday afternoon, the National Weather Service raised the prospect that the approaching convective system might in fact qualify as a derecho. The media, including the Capital Weather Gang, broadcast this message.

Ever since the highly destructive 2012 Ohio Valley-Mid Atlantic derecho, the use of the “D Word” has become somewhat unsettling, if not downright frightening, to Washingtonians. It is important to note, however, that not all bow-shaped lines of thunderstorms — even those containing damaging winds — meet the definition of a derecho.

That definition, in fact, has been in flux, within the scientific and forecasting communities. The original criteria required a continuous path of wind damage exceeding 240 miles, widespread gusts exceeding 58 mph (what the NWS defines as a “severe gust”), and two or more widely spaced wind observations exceeding 74 mph. (This last criteria is not universally used by everyone). There are now suggestions to abandon this definition in favor of one that instead reflects structural characteristics and evolution based on radar observations.

Strictly speaking, Monday’s bow echo initiated over southeastern Ohio, and continued all the way to the Chesapeake Bay, hours later. Length criteria would be met along this trajectory. However, wind damage reports were spotty all along the track. As you can see from the wind damage reports shown below, they are patchy but not widespread.


(National Weather Service, adapted by CWG)

Whether Monday’s storm qualifies as a derecho, even as a low-end derecho, is debatable, and we are not inclined to label it as such. Yes, it was sudden and impressive, and it struck during a vulnerable time (rush hour). Lacking the hard observations required by the original scientific definition, it fell just short in our estimation.

A supercell embedded in the storm

Returning to the damage map (above), you will notice a concentrated corridor of large hail, wind damage and tornado warnings extending from the West Virginia panhandle, through Loudoun and Fairfax Counties. Along this northwest to southeast track, an especially intense cell — possibly a supercell — exhibited signs of rotation and also Doppler-derived wind speeds approaching 75-80 mph a few thousand feet above the ground.

On radar, this singular cell really stood out (magenta blob over the West Virginia panhandle). It is likely that rotating air concentrated along the frontal boundary served as a source of spin for this storm.


Radar image showing a likely supercell (magenta blob) on the northern end of the bowing line. (weathertap.com)

Bow echoes at times contain vortices, including a type called a mesocyclone contained in classic, isolated supercells. Bow echoes occasionally drop tornadoes, accounting for as many as 20 percent of all U.S. tornadoes.  These tornadoes, however, tend to distribute toward the weaker end of the spectrum. Although there was a funnel cloud sighting, any tornado, Monday, in Loudoun and Fairfax Counties has not yet been confirmed by the NWS.

A bow echo that moved along a similar track to Monday’s event, on June 13, 2013, also contained a supercell-like structure on its northern end, spawning the 17-mile long “ICC Tornado” in Montgomery County (rated EF0).

Whether a tornado was confirmed, this intense northern bookend along the fierce line of storms took its toll.

Below are photos of the associated wind damage and hail …

Capital Weather Gang’s Jason Samenow contributed to this report.