When people think tornado, the first thing that comes to mind is the strong or violent tornado of the Great Plains, generated by supercell thunderstorms. But nature manufactures tornadoes in many ways, and another relatively ubiquitous type are “squall line tornadoes,” generated along the front edge of a bowing or arc-shaped complex of thunderstorms.
Multiple such squall line tornadoes touched down in the Washington area Thursday.
According to one research study, these types of squall lines, called quasi-linear convective systems (QLCS), generate 20 percent of all tornadoes in the U.S. They are typically weaker, smaller and shorter-lived than the classic supercell-type tornado, which is manufactured by a spiral or “helical” updraft called a mesocyclone.
The squall line responsible for Thursday’s wind damage in D.C. approached the city from the southwest, as shown in the figure below.
This was a very curious line of storms, for several reasons. For one, it lacked lightning. Two, it had a very wavy structure, with multiple arc-shaped segments separated by sharp indentations or notches. Three, it formed in an environment that was only marginally unstable, but that lack of instability was compensated by extremely vigorous ascent of air beneath the jet stream.
A packet of upper-atmosphere spin energy, called a vorticity maximum, was surging into the Washington region, and the squall line was the direct reflection of violently rising air along the leading edge of this energy packet.
The indentations along this wavy squall line likely marked the location of small vortices, which developed in response to the extreme wind shear (increase in wind speed with altitude) overspreading our region.
The squall line had every appearance of a QLCS from the standpoint of weather radar. This is why the National Weather Service issued a tornado warning for a segment of Northern Virginia, and central Maryland, in the path of this storm. Meteorologists are only now discovering the mechanisms by which QLCS tornadoes develop, and it is a very different process than the classic supercell-type tornado.
There are real challenges behind detection of QLCS tornadoes; their parent circulations, called misocyclones, are smaller, shallower and shorter-lived than the vigorous tornado-spawning mesocyclones of supercells. They more easily elude detection using Doppler radar, especially at long distances from the radar. And they are less likely to be identified visually, as they are often embedded in heavy rain along the leading convective line of QLCSs. Their strong, rotary winds often become embedded in the powerful straight-line winds of downbursts, making a post-mortem assessment even more difficult.
The figure below shows a Doppler radar snapshot of the QLCS as it swept across the District, from southwest to northeast, at 2:42 p.m. The scanning radar was located at Andrews Air Force Base, to the southeast of D.C. This is a highly sensitive radar, specifically attuned to detecting subtle changes in horizontal wind speed and direction (wind shear) close to the air terminal. The radar resolved two misocyclonic circulations, shown by the white arrows in the figure.
The counterclockwise-rotating misocyclones were detected along the front edge of the QLCS, as expected. They are recognized as velocity couplets — small regions of strong outbound (red color) airflow adjacent to inbound (green color) radial velocity. While these rotating air columns were detected over a thousand feet above ground, within the cloud layer, their vortical circulations likely extended to the ground, becoming amplified as very weak and transient tornadoes, creating spotty wind damage across the District.
The characteristic funnel cloud of a tornado was missing in many of the videos we examined. But in the case of the likely tornado that hit the Tidal Basin and H St. Corridor, we could see (in one instance) the spinning column of water spray on the river, and rotation in the cloud base.
Why wasn’t there a white funnel visible from the cloud to the ground like you often see in tornadoes? The weak nature of the rotating air column meant that the core pressure drop was insufficient to cause enough expansional cooling for a condensation funnel cloud to form.