The storm, which contained a tornado, passes through Columbia, Md., on Thursday. (James Willinghan/Twitter)

The National Weather Service confirmed a tornado touched down around Columbia, Md., on Thursday, rated EF1 on the 0-to-5 scale for tornado intensity. The twister snapped trees and peeled part of a roof off a building.

Compared with the common, tornado-breeding supercell of the Plains and mid-South, the Columbia tornado was produced by a storm of a different kind.

It’s called a quasi-linear convective system (QLCS), a kind of wavy, vortex-riddled squall line of thunderstorms. On radar, the system has a number of bowing segments and indentations or “clefts.” Dangerous straight-line winds, including downbursts, often blast out of the bows; tornado-generating vortices hide in the clefts.

Shown below is a snapshot of Thursday’s QLCS as it approached the Washington region.


A QLCS squall line approaching Washington region. (RadarScope.com)

The image shows the approaching storm at 3:32 p.m. Note the two prominent bows (white arrows) — one over Laurel, Md., and the other near Oakton, Va. Clefts or indentations along the front edge are found just north of Columbia, Md., and southeast of Reston, Va. (pink arrows).

Prominent indentations can also be found along the back of the line, such as the large one east of Germantown, Md., and a smaller one over Centreville, Va. (blue arrows). Those back-edge notches are thought to be caused by small jets of wind surging into the rear of the storm line.

Taken together, these features seem to form a series of “wavy S’s,” and when meteorologists see these attributes, they know the potential for locally damaging winds is heightened.

The schematic below illustrates the evolution of the scroll-like characteristics embedded within a QLCS, revealing they are caused by numerous small vortices, or mesocyclones. These areas of spin develop when the wind shear (change in wind speed with altitude) is very strong, as it was over Washington on Thursday.


A schematic evolution of a QLCS-type squall line.

There is tremendous “spin energy” in that shear, which is amplified (concentrated) by the intense updrafts contained in the QLCS. Thursday’s very strong instability (warming of the ground by the sun) stoked these convective cells to great intensity. The critical interplay between strong wind shear and instability is something meteorologists know to watch for.

The final cartoon graphic in the image sequence above shows how the northern end of each bowing segment can enlarge over time, as a prominent counterclockwise (cyclonic) vortex takes over. Now, take a look at the Washington QLCS as it appeared east of the city, at 4:05 p.m. (below). Note the very large “curls” evident in the radar pattern, near Glen Burnie, Md., and southwest of Bowie, Md.


A radar snapshot of the QLCS storm after it crossed through Washington.

Let’s zoom into two of the mesocyclones in Thursday’s QLCS. Recall that the one near Columbia was contained in a cleft or indentation on the north end of the line (top image). Instead of conventional radar reflectivity (which measures rain intensity), we switch our view to Doppler-derived winds.

Meteorologists use red-orange colors to denote winds blowing away from the radar (located at Dulles International Airport), and blue-green colors for winds flowing toward the radar.


A Doppler image zooming on mesocyclone near Columbia, Md. The arrows are the author's interpretation of airflow. (Adapted from RadarScope.com)

At 3:28 p.m. (image shown above), the closely opposed outbound and inbound flows form a counterclockwise, swirl-like feature just west of Columbia. The swirl is the hallmark of a mesocyclone (parent or precursor of a tornado), and I’ve placed a marker indicating “75 mph” outbound wind speed. In fact, this pocket of high wind from the west caused a prominent bulge or bowing of the storm line, while a cleft or indentation — generated by opposing winds from the east — notched into the line just northwest of Columbia.

Imagine the mesocyclone as a giant cylinder of whirling air, extending through the lower reaches of the storm clouds, to just above the surface. But it’s several miles in diameter, and is not a tornado. The majority of mesocyclones, in fact, do not give rise to tornadoes. But this one near Columbia did.

By 3:34 p.m., an EF1 tornado had developed within the larger mesocyclone, as the larger vortex moved rapidly eastward. The tornado was on the ground for only a few minutes. It’s typical of QLCS-generated tornadoes — which tend to be low-end intensity (EFO and EF1) and short-lived.

In contrast, a great many strong (EF2 and EF3) — and all violent (EF4 and EF5) — tornadoes are generated by lone supercell thunderstorms, which contain much stronger mesocyclones that may persist for hours.

There was another pocket of wind damage, one over Falls Church-Arlington in Virginia. The Doppler image shown below was obtained at 3:42 p.m.


A Doppler image of a downburst/weak mesocyclone over Arlington and Washington. Arrows are the author's interpretation of airflow. (Adapted from RadarScope.com)

This could have been part of a very asymmetric circulation, with a much stronger, westerly surge of wind on its southern edge, and only weak “notching” from easterly winds on the north side. But another interpretation of the wind pocket (marked as “84 mph”) is that of a downburst.

In contrast to tornadoes, downbursts are intense, sinking pockets of air that create a violent outburst of wind along the ground. The motion is down, outward (divergent) and straight-line, whereas tornadoes are up, inward (convergent) and rotary.

It’s possible that a portion of the mesocyclone also extended to the surface, producing a swath of straight-line wind damage.

In either case, downbursts and descending mesocyclones can generate wind speeds of 70-plus mph (and downbursts can exceed 100 mph), easily on par with weak tornadoes.