More than 13,000 customers lost power, mostly in the eastern part of the District and adjoining sections of Maryland.
The forecast for the Washington region Monday evening did not include severe weather. While Capital Weather Gang had predicted widely scattered storms, which could be locally heavy, their intensity exceeded expectations. The National Weather Service hadn’t placed the region in an elevated threat zone for severe storms, and no watches were in effect.
But storms that formed in Frederick and northern Loudoun County erupted as they swept southeastward on the District’s hottest day of the year; the high was 97 degrees. The storms became particularly intense in the corridor from near Wheaton and Silver Spring south through Takoma Park and just east of downtown Washington. A weather station near Catholic University clocked a wind gust to 67 mph.
D.C. Fire reported that three homes were ordered vacated because of tree damage, displacing eight adults and one child.
Here are some eyewitness accounts of the storm as it roared through:
“I was driving into my neighborhood and dodging branches and power lines. It was terrifying,” tweeted Brigid Howe, describing the scene in Wheaton.
“From Columbia Heights. I haven’t seen anything like this since the dc derecho [in June 2012]. Already see a large downed tree branch and my roof is leaking,” tweeted Lori Olson.
“As the storm pushed in about 15 mins ago, watched a tree get blown sideways and snap off the top in Capitol Hill neighborhood near Union Station,” tweeted Mark Bauer.
The National Weather Service serving the Washington and Baltimore region received more than 50 reports of severe weather, mostly of downed trees due to localized downbursts of wind, often referred to by meteorologists as microbursts.
How it happened
The storms initially fired off in the vicinity of a diffuse cold front in the region advancing to the southeast.
The particularly intense cell that downed a large number of trees from around eastern Montgomery County into the District is shown in the radar snapshot below, shortly after 7 p.m. The yellow polygons on the figure indicate the outline of severe thunderstorm warnings issued by the Weather Service.
The trigger for this intense cell was not captured by the highest-resolution, hourly updated forecast models.
In the loop below, note the approach of two arc-shaped air mass boundaries, indicated by radar, just to the northwest of D.C. You will note the heftier of the two boundaries sweeping from east to west, slamming into a second boundary working its way down from the northwest.
The damaging storm cell erupted in the zone of collision between these two boundaries, which behaved like very localized cold fronts. In an atmosphere that was unstable because of high humidity and hot temperatures, all that was needed was “convergence,” or the squeezing together of two colliding air masses, for storms to erupt.
The air mass boundary surging from the east was probably a pool of cool air off the Chesapeake Bay known as a bay freeze front (shown below on radar at 5 p.m.); the northern boundary, which sagged toward the southeast, was triggered by coalescing downdrafts flowing out of storm cells over Montgomery County.
A few minutes after these cool, dense air masses collided, up popped the solitary storm cell that generated the locally damaging winds over far southern Montgomery County and the northern part of the District.
The storm cell’s intensity was aided by a localized pocket of wind shear (strong increase in wind speeds with altitude) centered over the region, just as the air mass boundaries collided. The wind shear was able to tilt the storm updraft up and away from the downdraft, ensuring the drafts did not interfere with and destroy each other.
Another factor contributing to damaging wind gusts was the difference in temperature between rising, hot air in cloud updrafts and descending, chilly air in cloud downdrafts. In the lowest few thousand feet of atmosphere, the difference was very pronounced Monday evening, allowing the downrush of air to accelerate to high speed as it impacted the ground.
It is noteworthy that the wind damage zone developed a significant distance north of a severe thunderstorm watch issued by the Storm Prediction Center, the northern boundary of which was located about 50 miles south of the District (figure below).
The logic for placing the watch south of D.C. was sound, as this was the location of the most unstable air mass into which the southeast-sagging cold front was advancing, scooping up and lifting very buoyant air.
Indeed, wind damage events occurred within that severe thunderstorm watch. But leave it to the vagaries of the bay breeze front to create a tiny pocket of mayhem over the District, which forecasters were unable to identify ahead of time.