A weak but damaging tornado carved a four-mile path through Reston just before 9 p.m. Friday night, leaving behind downed trees, a smashed vehicle and a townhouse that had to be condemned.

The National Weather Service surveyed the damage from the twister Saturday and estimated the tornado’s peak winds at 70 mph. While these winds were strong enough to uproot and shear the tops off trees, the twister rated at the low end of the spectrum for tornado intensity.

Tornadoes are rated on Enhanced Fujita (EF) scale that ranges from 0 to 5, and the Weather Service put Friday night’s storm at EF0.

Along its four-mile path, the Weather Service’s report on the storm said tornado damage was “discontinuous.” In some areas, trees were toppled, while other areas were left untouched.

The Weather Service noted “isolated” tree damage near the U.S. Geological Survey complex and just north of the Reston Hospital Center. After passing a Trader Joe’s grocery store and destroying an outdoor shed, the storm took down a tall, two-foot diameter tree that “crashed through the upper floor of a townhouse on Quietree Drive,” which led officials to condemn the property, the Weather Service report said.


A tornado in Reston, Va., toppled the tree that smashed this townhouse. (John Keady)

The storm downed several more large trees in Reston neighborhoods, including one 100 feet tall that smashed an unoccupied vehicle along Center Harbor Road near Reston Parkway.

In a 12th floor apartment of a high-rise at Reston Town Center, resident Andrew Chapman “heard a strange loud rushing sound," moments after his phone alerted him of the tornado warning.

“[The sound] was both familiar and unlikely anything I’d heard before, almost like the sound of a breaking wave at the beach but not stopping,” he said. “Within less than a minute, as it was getting louder, we had a strong sense it really was a tornado.”

Chapman sheltered in the building’s underground garage and, having left one of his apartment windows open, returned to mess (as shown in the photo below).


Apartment at Reston Town Center had belongings blown around as a tornado passed nearby Friday night. (Andrew Chapman)

“[T]he tornado literally passed by right outside our window!,” he said.

Police and fire officials said there were no reported injuries, but the storm caused plenty of disruption.

“Our officers responded to a ton of roadblocks,” meaning streets filled with tree branches, power lines and other debris, said Sgt. James Curry, a police spokesman. He said, “There were a couple of close calls” in which falling debris narrowly missed parked vehicles and homes, but that property damage was not widespread.

The twister was short-lived and fast-moving, lasting just five minutes as it raced north to northeast at 50 mph.

How it happened

About 8:30 p.m. Friday, the approaching squall line ahead of the larger storm’s cold front distorted into an S shape across Northern Virginia. Part of that S began rapidly bowing northward into an arc of intense storms, crossing Manassas and Centreville. This arc of storms is shown in the radar image below.


Radar view of storms. Left panel shows precipitation intensity. Right panel is an estimate of wind velocities. (RadarScope)

Gusts along the bow were significant (green colors in the right panel of the figure above) but contained within was a small pocket of extreme gusts of nearly 90 mph (blue colors) that were not far above the ground. This possible downburst was east of Centreville and south of Chantilly, lasting just minutes and only a few miles across. Downbursts are often contained within bow echoes and typically located near the apex of the bow.

A few minutes later, as it surged rapidly north, the bow echo began to break up. What emerged about 9 p.m. was a transient supercell or rotating storm, shown in the figure below, which zoomed in on the Reston area. The southern end of the supercell contained a hooklike appendage encompassing Reston.


Radar showing the tornado thunderstorm near Reston just before 9 p.m. Friday night. The left panel shows precipitation intensity and a hook-shaped pattern near Reston, sometimes indicative of a tornado. The right panel shows a velocity couplet near the area with winds blowing away from the radar (in red) and toward the radar (in green), also often indicative of a tornado. (RadarScope)

The Doppler radar winds revealed a counterclockwise circulation (mesocyclone) within the hook and over Reston — shown by the red and green colors in proximity. Red colors are winds racing away from the Doppler, green shows wind blowing toward the radar. Together they complete a counterclockwise circulation very close to the ground. The Reston tornado developed within this mesocyclone.

The rapid increase in southerly winds with altitude Friday evening, called a wind shear, created spin in the atmosphere — giant, invisible horizontal “rollers” of wind. Updrafts in the convective clouds bent parts of these rollers into the vertical, changing the axis of rotation from horizontal to vertical. This is one way the mesocyclone could have formed.

Photos

The Washington Post’s Paul Duggan contributed to this report.