Severe storms bring damage, possible tornadoes in Fairfax County

WEATHER NEWS: Severe storms bring damage, possible tornadoes in Fairfax County

The storm system responsible for the potential twisters is the same one that unleashed dozens of tornadoes across the South on Wednesday and into Thursday morning, resulting in numerous injuries and at least two fatalities.

Ahead of the possible tornadoes, the National Weather Service had issued a severe thunderstorm watch early Thursday afternoon highlighting the potential for damaging straight-line winds and mentioning the possibility of a couple tornadoes. The Weather Service had placed the Washington region in an “enhanced risk” zone for severe thunderstorms.

Unlike in classic summertime tornado setups, which are fueled by high heat and humidity, instability or “fuel” for storms was limited. It was only 70 degrees at the time of the storm at Washington Dulles International Airport. That said, there was an extreme amount of shear, or a change in wind speed and direction with altitude. That allowed for storms to rotate.

How the potentially tornadic storm developed

A storm began looking ominous in eastern Prince William and western Fairfax counties just before 8 p.m. It was riding a differential heating boundary, or an interface between slightly cooler surface temperatures to the north and acutely milder air to the south. Boundaries are like railroad tracks that storms can chug along; sometimes, storms consume extra twist along these boundaries.

By shortly after 8 p.m., the storm was showing the characteristics of a supercell, or rotating thunderstorm. It had a circulating updraft and was located at the southern end of a line of storms. That usually gives a storm room to strengthen, since there’s nothing to the south to impede its “inflow” of warm southerly winds.

The storm rode directly over Interstate 66. Although rotation was evident on radar from the Weather Service office in Sterling, it didn’t look terribly tight. Still, the National Weather Service issued a precautionary severe thunderstorm warning at 8:18 p.m.

That’s about the time when the first tornado may have touched down.

Possible touchdown near Centreville

Despite unimpressive rotation from the Weather Service’s WSR-88D radar — a more powerful device that offers slightly coarser resolution — a high-resolution and ultrasensitive terminal Doppler radar at Dulles painted a different story. It showed a tight couplet of spin between Chantilly and Centreville.

In the radar image presented below, green marks winds heading toward the radar, and red away.

The Weather Service issued a tornado warning for Fairfax County seconds later, though a brief tornado already may have been on the ground. The couplet was strong for two scans, and there are indications that debris may have been lofted into the atmosphere. That can be seen on the bottom right of four-panel image above. A low “correlation coefficient,” marked in blue, is usually an indicator of spiky objects in the atmosphere that aren’t rain or hail. That can be commensurate with radar debris.

Karyn Miller, a Capital Weather Gang follower on Facebook who lives in the Sully Station area of Centreville, commented that a possible circulation “ripped some … siding off the chimney and a couple other small places” of her home. That damage would be suggestive of an EF0 tornado or straight-line winds in the 70-80 mph range. Her propane grill was moved four feet across the deck.

Possible touchdown No. 2 near Tysons Corner

A second probable touchdown occurred about 8:39 p.m. in Tysons Corner. That’s where radar showed a tight “spectrum width” signature, revealing turbulent motions within pixels on the radar. A similarly tight couplet also briefly materialized on velocity mode.

The above image shows fine scale radar structure of the possible tornado signature. The top row shows an “inflow cleft” where a rapidly developing, small counterclockwise circulation along the storm’s leading edge, draws in warm, moist air. The bottom row shows continuation of the notch structure several minutes later, and at this time a subtle Doppler signature of rotation develops.

One video on social media (caution: strong language) captured what appears to be a 30- to 50-foot-wide funnel moving through the Tysons commercial district, flanked by strong westerly “rear flank downdraft” winds on the south side of the circulation. That’s where cool air wraps counterclockwise around a circulation.

Debris can also be seen falling from above and becoming entrained in the apparent vortex, indicative of a probable EF0 or EF1 tornado. Winds were probably below 90 mph. The damage in the rear flank downdraft from straight-line winds was probably equivalent in magnitude to that caused by the tornado.

Damage was reported at multiple gas stations along Chain Bridge Road. The Sunoco Station off International Drive suffered serious damage to its canopy, which was toppled, and the Mobil station next door lost at least three window panes and suffered damage to an awning. That would suggest a path length of about a quarter-mile.

Possible tornado debris landed on the Silver Line Metro track near Tysons, causing rail delays and single-tracking, according to a tweet from WMATA.

Tornadoes in March in the Washington region are not terribly common. On average, the region sees about one tornado every decade in March, according to Ian Livingston, a Capital Weather Gang contributor.

Jason Samenow and Capital Weather Gang severe weather expert Jeff Halverson contributed to this report.

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