Between 10:30 p.m. and 12 a.m. Sunday night, a surge of wind blasted through the D.C. area. Trees came down, structures were damaged, and tens of thousands lost power.
“The winds are here out of nowhere,” Charles Baserap of Rockville, Md., wrote on Facebook as the burst arrived. “Worst gust I’ve heard hit the house since the [June 2012] derecho to be honest. And the length of sustainment is impressive. A constant roar …”
Other eyewitnesses called the winds “unnerving,” “wicked,” “scary,” “very intense,” “crazy” and “no joke.”
Wind gusts of over 50 mph were widespread, with localized readings over 65 mph.
Reagan National Airport clocked a peak wind gust of 66 mph, the strongest since the 70 mph gust during the derecho of June 29, 2012. It was the airport’s third-highest gust since 2001, according to the National Weather Service.
Baltimore-Washington International Marshall Airport reported a peak gust of 51 mph, while Dulles International Airport registered a gust of 58 mph.
The region’s highest gust, 72 mph, was recorded at Andrews Air Force Base, and was just 2 mph shy of hurricane force.
“It was insane here in Herndon,” Chris Wunsch wrote on Facebook. “After having our house damaged by falling trees during the last big windstorm on November 20th 2016, I can say this seemed worse. Higher sustained winds and they lasted a lot longer.”
The wall of wind was initiated by an arc-shaped line of showers and thunderstorms that developed in western Pennsylvania. Also known as a squall line, it charged southeastward and left behind nearly 80 reports of damaging winds along its path.
On radar, the squall line did not appear severe. Its rains were light and its so-called thunderstorms produced little in the way of actual lightning and thunder. But its violent winds prompted the National Weather Service office serving the D.C. area to issue six severe thunderstorm warnings. They all verified because winds reached at least 58 mph — the established criteria for “severe” wind gusts.
This line formed ahead of a vigorous cold front advancing across the Ohio Valley. The amount of instability supporting the squall line was extremely limited. Its development was nothing like a typical line of summer storms, in which an unstable atmosphere leads to strong updrafts (i.e. “hot air rises”) and towering storm clouds over 40,000 feet high.
Instead, Sunday night’s storms were much more shallow or “low-topped” but still contained fast-rising air. The powerful front focused rising air in narrow conduits and strong high-altitude winds helped sustain the storms as they raced southeastward.
Importantly, the winds just a few thousand feet above the ground were roaring in from the west at nearly 70 mph. The extreme momentum of this fast flow was effectively transported to the ground by the squall line’s downdrafts. This helped to build a ground-level surge of wind, called a gust front, that swept across our region with widespread gusts in the 50-60 mph range.
Additional processes at the jet stream level likely helped generate and sustain the squall line. A pocket of exceptionally fast-spinning air, called a “vorticity maximum,” was crossing the Ohio Valley. Meteorological principles predict vigorous uplift of air along the leading (advancing) edge of these vorticity maxima. Analyses of last night’s squall line place it exactly along the advancing edge of this pocket of energy.
The squall line was thus energized by dynamical uplift both from below and from above, making up for the lack of hot and humid air, the characteristic ingredient in intense summertime storms.
Strong winds continued to lash the Washington region Monday as a storm that developed along the front exploded east of New England. But the worst of the winds occurred with the squall line passage Sunday night.
Angela Fritz contributed to this post.