Both storms developed in a similar type of severe thunderstorm environment. The meteorology is complex but fascinating. Let’s take a closer look at Monday’s storm.
The weather radar view
The nearest National Weather Service radar to capture the tornado (which occurred between 1:30 and 1:40 p.m.) is in Dover, Del., to the north of the storm. This is a Doppler radar, meaning it can detect air flowing toward or away from the radar.
As shown in the figure above, when a “velocity couplet” is detected — that is, a region of inbound airflow adjacent to an outbound region — one can infer the presence of a mesocyclone with counterclockwise rotation. A mesocyclone is a two- to three-mile-wide vortex, rotating within the thunderstorm’s updraft, that sometimes initiates a tornado.
Located even closer than Dover is the NASA Wallops Flight Facility’s NPOL (“NASA Polarimetric”) research radar. Meteorologists at this site were collecting data on the storm, and they provided this astounding, high resolution image.
The panel on the left is a conventional radar view of the storm, showing radar reflectivity (the amount of energy backscattered by concentrated rain drops). The red core sports a bit of a “hook echo” which is where rain is wrapping into the mesocyclone’s circulation.
The panel on the right is the Doppler view, taken at the same time; yellow and red colors represent air flowing away from the radar, green toward the unit. Again, note the velocity couplet, here very tightly resolved. This scan was completed about 30 minutes before the one shown in the figure above, suggesting that this supercell storm had some staying power.
How did this storm form?
At least 24 hours beforehand, meteorologists thought that the region southeast of D.C. might be ripe for severe thunderstorms, specifically a type of storm called a supercell, that sometimes initiates tornadoes.
In the Mid-Atlantic, with rare exceptions, most supercells are fairly short-lived and small, often dubbed “mini-supercells” (compared to their larger cousins over the Great Plains and Mid-South). Many never produce tornadoes; large hail is the more likely severe weather type.
But yesterday’s setup had recognized potential for one or more tornadic supercells. Meteorologists look at the amount of instability in the atmosphere, along with deep-layer wind shear (change in winds with altitude from the surface up to about four miles), and also a measure of low-level shear involving both an increase in speed and clockwise turning of the wind direction.
The next figure shows the basic setup in terms of surface weather features. The prominent feature was a warm front (red, scalloped lines) south of D.C. but extending across the northern portion of the Bay.
Not shown in the figure, and located farther west, was a developing low-pressure center, over central Virginia. This growing storm was drawing a plume of warm and humid (unstable) air northward near the ground across much of the eastern Mid-Atlantic (red curving arrows).
The next figure illustrates the northward surge of this tropical-like air, up and over Salisbury (red diamond).
Here, the location of the low-pressure region can now be seen (giant red “L”). There was a fair amount of buoyant energy in this plume, feeding storm updrafts. The numbers shown here reveal the calculated energy content, in Joules/kg, for a shallow air layer above the ground. Other measures, which calculate the energy content at the heated surface, suggest numbers as high as 2,500 Joules/kg. The faster a storm’s updraft can be made to rise, the more tightly stretched and contracted (faster spinning) will be a mesocyclone developing within the updraft.
The updraft’s source of spin is rooted in the strong, deep layer of wind as shown in the next figure.
Air moving significantly faster aloft than at the ground gets the air overturning, rolling over on itself. A strong cloud updraft rising into the windy layer “loops” up these horizontal vortex tubes or “rollers” and reorients them into the vertical, where they transform into one or more mesocyclones. The pocket of really intense shear Aug. 7 was surprisingly compact, and also surprisingly strong (nearly 70 mph of shear) for this time of year. It was accompanying a “shortwave” or ripple in the jet stream as it traversed the Eastern Shore.
The shortwave also created large regions of spin at different levels in the atmosphere (the spin covers much larger regions than single thunderstorm cells; do not conflate with a mesocyclone), and a basic tenet of atmospheric dynamics states that this situation can induce air to rise.
Tuesday’s storms over the Eastern Shore were subject to very strong “dynamic lift,” and the intensity of jet stream spin is denoted by levels of green shading in the image.
A red “X” marks the center of rotation in the image above. Along and ahead of the “X,” air was forced to vigorously ascend — basically over the entire Eastern Shore. This uplift helped to further destabilize and moisten the atmosphere, pushing individual thunderstorm cells closer to severe levels.
The final critical ingredient for a tornado is difficult to visualize: Within the supercell thunderstorm, with its rotating updraft (mesocyclone), some process or set of processes must shrink down and concentrate part of that spin, very close to the ground. Understanding exactly what this mechanism is remains a Holy Grail of meteorology. A larger mesocyclone is often thought to be a necessary, but insufficient, condition for tornadoes. In and of itself, a mesocyclone is NOT a tornado, and about two thirds of all mesocyclones fail to produce tornadoes.
Take a look back at the earlier figure depicting the warm front. These surface boundaries mark strong gradients between cool and warm air. As warm air rises and cool air descends, air begins to spin along (parallel to) the boundary, at low levels. A nearby supercell can loop up tubes of this spinning air, right along the ground, which helps build the mesocyclone toward the surface and intensify. This likely helps with the final stages of tornado formation. In fact, a nearby warm front is a classic ingredient for tornadic supercells in our region of the Mid-Atlantic.