How Wednesday’s historic Maryland tornado outbreak happened

That supercell produced tornadoes near Darnestown, Poolesville, Gaithersburg, Olney, Columbia and Baltimore. Eyewitness footage showed that some of these twisters were quite large and more typical of what’s common in the Plains and southern United States. Drone video even revealed a multivortex tornado at times, meaning the twister had multiple swirls that orbited around the central funnel.

The National Weather Service received 16 tornado reports from Maryland, one from Loudoun County and four from eastern West Virginia.

It is not clear how many tornadoes occurred or how strong they were; meteorologists at the National Weather Service will survey damage beginning Thursday to determine this.

But the swarm of twisters came somewhat as surprise. Although the Weather Service forecast office serving the Washington region had mentioned that “a tornado or two” was possible in its Wednesday morning discussion, the agency’s Storm Prediction Center based in Norman, Okla., never issued a severe thunderstorm or tornado watch. It only placed the area in a Level 1 out 5 “marginal risk” zone for severe storms, while highlighting a low risk of twisters.

The Capital Weather Gang mentioned the slight possibility of tornadoes on social media Wednesday morning as well as in its early afternoon forecast update, but no forecaster we know of anticipated one of the most prolific tornado outbreaks in recent memory.

Here’s what happened.

The elements that came together

The Washington-Baltimore region was in a tiny region where the necessary tornado-producing elements converged for a short window. What ensued was a kind of “micro-outbreak” that was very difficult to forecast given the sparse nature of our data observation networks.

The following diagram is our attempt to synthesize the key elements that came together.

A key feature that helped the supercell storms form was a warm front, which was slowly tracking northward across the region throughout the day before stalling along the Mason-Dixon Line (thick red boundary line). It drew a very humid, warm and unstable air mass (maroon oval) into our area. Thick morning overcast gave way to enough breaks in the cloud canopy for the sun’s warmth to stoke instability further, and additional “storm fuel” arrived from the south. By late afternoon, we had an abundance of buoyant energy in the atmosphere.

The genesis of rotation in thunderstorms lies in the strong turning and speeding up of airflow in the lower atmosphere. The combination of a tiny pocket of deep wind shear (magenta stippled region) and low-level spin potential — called helicity (blue shaded pocket) — was perfectly collocated within the nose of the most unstable air. Much of the spin originated from the warm front, which had winds of strongly opposing directions on either side of its boundary. This attribute played a critical role in the storm outbreak. Warm fronts can be underrated in their potential to give rise to twisters.

Finally, the chance coincidence of a pocket of tightly wound, spinning air in the middle atmosphere set the stage for intensely rising air adding oomph to cloud updrafts. The bull’s eye of green colors and “X” marks this whirling pocket, which originated far to the west as a remnant of an earlier thunderstorm complex. We call this zone a “mesoscale convective vortex.” Drifting along in the atmosphere’s flow, the former storm complex helped reignite the atmosphere hours later, much farther to the east.

How the storms evolved

The supercell storms, containing rotating updrafts called a mesocyclone, formed and tracked along the warm front zone.

The southernmost supercell in Montgomery County displayed a prominent hook on radar — a swirling region of heavy rain wrapping around the mesocyclone — as shown in the visualization below:

Such hooks are often a telltale sign of a tornadic thunderstorm.

This rotating cell tracked out of Poolesville and into Gaithersburg, then to Olney. It appears that it was a “cyclic” tornado producer, responsible for repeated generation of tornadoes, or a family, spanning a long path from Northern Virginia’s Loudoun County to Baltimore County. Such a prolonged twister-producing storm — traveling about 40 miles before continuing on into northeast Maryland — is unusual in the Mid-Atlantic.

While the Montgomery County supercell was particularly prominent, it wasn’t the only one. The tracks of individual mesocyclones — which produced additional tornadoes — are shown in the image below.

Let’s drill down into the Montgomery County supercell when it was perhaps at the height of its tornadic intensity, near Gaithersburg. This was an exceptional tornado producer for our region, in that the Doppler radar at Dulles Airport detected debris lofted as high as 11,000 feet.

The panel below shows four types of radar views of this cell, at 7:20 p.m. The upper left is standard “radar reflectivity” revealing rain intensity and, most importantly here, a prominent hook echo.

The upper right provides a key signature which shows the direction and speed of winds in the storm cell. The circled region points to a small but potent embedded counterclockwise circulation within the hook echo. This is called the “velocity couplet” — indicating by adjacent red and green patches where winds are blowing toward and away from the radar — and marks the location of the mesocyclone and embedded tornado.

The bottom two panels present something rarely seen around here: indications of a “tornado debris signature, ” or cloud of lofted debris detected by the radar. This signature is perfectly correlated in time and space with the hook echo and velocity couplet. This trifecta of radar presentations is the hallmark of a radar-detected tornado on the ground.

And for this reason, with a known tornado tracking through a densely populated region, the D.C.-Baltimore office of the Weather Service issued its first-ever “particularly dangerous situation” tornado warning for Montgomery County. These types of warnings were first implemented in the mid-2010s to call special attention to the most ominous, large tornadoes.

How rare was this outbreak?

The Weather Service serving the Washington region issued 22 tornado warnings Wednesday, which is fourth-most since 1986. Tornadoes are most common in our areas between May and September, so this event occurred on schedule.

Although it will take at least a day or two for the Weather Service to assess the damage and assign ratings to the twisters, all indications are that this was a very significant event in Maryland tornado history.

Only seven days since 1950 have witnessed half a dozen or more tornadoes in the state. While many twisters Wednesday came from the same storm, and the preliminary count of 16 in Maryland may ultimately be reduced, it’s probable that the event will rank among the top 10.

On July 7, 1994, 14 tornadoes touched down in Maryland, the most on record. More recently, there were 10 on Aug. 4, 2020, and 13 on June 1, 2012.

For the District, Virginia and Maryland combined, the biggest tornado day was associated with Hurricane Ivan in 2004, when 42 tornadoes were confirmed across the three states. That day also featured a record 12 tornadoes rated at least 2 on the 0-to-5 scale for intensity across the region.

Video footage from Montgomery County showed a large, cone-shaped funnel near Darnestown and Poolesville — a powerful look that is more typical of tornado country west of the Appalachians. Initial analyses indicate that it’s possible that tornado could earn a rating of 2.

The last tornado rated at least a 2 to strike Maryland during June was in 1996.

Four tornadoes at least this strong were confirmed in the state on July 7, 1994, the most on record in a single day. Only five days have featured two or more tornadoes rated at least 2, most recently in August 2020.

Not since the infamous La Plata tornado April 28, 2002, has a tornado rated 3 or higher struck Maryland. The La Plata twister was rated a 4 and killed three people. Less than a year before that, a tornado rated a 3 struck College Park, causing multiple deaths.