At 9:42 p.m., a tornado ripped through parts of Baltimore and Dundalk, with peak winds of 105 mph, leaving a 2.5-mile-long trail of destruction and killing two workers at an Amazon distribution warehouse.
The Baltimore tornado was preceded by a tornado that roared through Mount Airy at 8:20 p.m., with peak winds of 100 mph. That tornado was on the ground for 4.7 miles.
Both tornadoes were rated EF-1 by the National Weather Service forecast office in Sterling, Va. The EF or Enhanced Fujita scale ranges from 0 through 5. EF-0 and EF-1 tornadoes are considered “weak” on the tornado intensity spectrum but, in some cases, are still devastating.
In addition to the tornadoes, torrential rain caused localized flooding in spots, and straight-line winds gusted to 40 to 50 mph in several locations, with a measured gust of 71 mph in Carroll County.
While forecasters had indicated that there was a somewhat elevated chance of severe weather Friday night, neither severe thunderstorm nor tornado watches were in effect in the hours before the storms struck.
The National Weather Service also did not issue a tornado warning for either twister, although it had issued a severe thunderstorm warning before the tornado touched down in Baltimore.
Chris Strong, warning coordination meteorologist at the National Weather Service office serving the Washington and Baltimore region, said his team is reviewing what happened Friday night and could not yet speak about why more warning wasn’t provided.
“This week we’re going to talk to the team to get their perspectives about what went wrong and what it could’ve done better,” Strong said.
The tornadoes were not spawned by discrete supercell thunderstorms, the most common source. Rather, they formed along an intense squall line known as a quasi-linear convective system (QLCS).
The term “quasi-linear” refers to the bowing or undulating appearance of the storm on weather radar, presenting as a series of wavelike lobes and clefts. The image below shows this extensive line crossing central Maryland around 9 p.m.
This QLCS was traveling toward the northeast at 30 to 40 mph, and was noteworthy for how little lightning it generated. Friday night, the normal warning cues of a potentially severe storm — including the strobe-like effect of lightning and thunder — were largely absent.
The sudden arrival of torrential rain, strong wind gusts and tornadoes, but without lightning, puts forecasters in a conundrum. Typically, one does not issue a severe thunderstorm warning for these conditions.
“It evolved very quickly,” Strong said. “It turned from a fairly benign rain event into a pretty intense QLCS event."
While the Weather Service tries to provide as much warning time as possible, Strong said tornadoes along these lines “invariably develop very quickly and fall apart very quickly.”
A special type of cold season squall line
During the cold season (October-March), a sub-variety of QLCS called a high shear, low CAPE (convective available potential energy) storm can sometimes occur in the Southeast and Mid-Atlantic. It was this type of squall line that swept through central Maryland on Friday evening.
“High shear” refers to the environmental winds in the atmosphere, which increase in speed with altitude. When the difference in winds over the lowest four miles reaches 40 to 50 mph, conditions become ripe for supercell thunderstorms. The wind shear value Friday evening was an astounding 70 mph.
But supercells did not develop, because the atmosphere was not unstable enough. An unstable atmosphere is one in which there is plenty of warm, buoyant air that rises to great depth. We measure the total buoyancy of air using the CAPE parameter, and values over 1,000 are generally needed to support thunderstorms. Severe thunderstorms become more likely with values over 2,000. Friday evening’s CAPE was just 200-300.
While these storm cells lacked updrafts driven by buoyant, unstable air, impressively strong dynamics in the middle and upper atmosphere helped to compensate. An amplifying wave of low pressure aloft (called a shortwave trough) in the jet stream caused air to rise vigorously over the region.
Air was also being lifted along a narrow zone ahead of a cold front, as shown below. An intensifying region of low pressure had formed along this front hours earlier and was tracking into the D.C. area during tornado time. Air converging into the low pressure area helped create further uplift of moist air.
The exceptionally strong wind shear, combined with vigorously rising air, set the stage for small regions of mid-level rotation to develop. These small vortices are called mesocyclones, and they initiated within portions of the QLCS line. These small vortices distort the shape of the line into a wavelike appearance.
Additionally, there was significant directional change in the winds with altitude, called veering of the winds, meaning they turn clockwise with altitude. Veering winds amp up the potential for tornado formation within preexisting small zones of spin. The surface low pressure system enhanced the turning of wind, from southeasterly at the ground, to southwesterly higher up in the atmosphere.
What we lacked in terms of instability was compensated by a combination of wind shear and low pressure. Had we experienced CAPEs in the range of 1,500-2,000, the outcome could very well have been a much larger tornado outbreak across the region.
The Mount Airy and Baltimore tornadoes in focus
The image below shows the radar structure over Mount Airy. As the left panel shows, there is no hook echo so typical of a supercell thunderstorm. Instead, there are numerous lobes of heavy rain, with clefts between the lobes. Mount Airy is near a particularly deep notch or indentation, where low-level winds stream into a mesocyclone.
That mesocyclone (embedded in heavy rain) is depicted in the right panel, which shows Doppler wind speeds at low levels in the cloud. Orange colors show fast flow from the southwest, green shading from the northeast. The strong directional change, which is counterclockwise, is termed a velocity couplet and identifies the parent mesocyclone responsible for spawning the Mount Airy tornado.
The next image below shows the same radar signatures in the vicinity of Baltimore and Dundalk during that tornado. The undulating characteristic is less apparent (left panel), but there is an intense convective cell (small purple region) just north of Dundalk, perhaps indicative of small hail and, thus, a robust storm updraft.
The Doppler velocity couplet is much larger and more diffuse, compared with Mount Airy — which would have made it even a trickier call as to whether issue a tornado warning. This may be due to the longer distance between Dundalk and the radar (near Dulles Airport), a factor that tends to smear out the details of these subtle areas of spin in the air.
Capital Weather Gang’s Ian Livingston contributed to this post.