Power lines hang over a street Monday in Stevensville, Md. (Jerry Jackson/The Baltimore Sun via AP)

The author, Matthew Cappucci, is a rising junior in atmospheric science at Harvard University.

Residents of Stevensville, in Queen Anne’s County, are picking up the pieces after a significant tornado swept through town early Monday morning, blasting coastal neighborhoods with 125 mph winds and causing at least one injury on its 2.37-mile path.

No one expected such a violent storm in the middle of the night. But with the benefit of hindsight, the ingredients were in place.

A rare middle-of-the-night twister

Nighttime tornadoes are rare in general, particularly along the East Coast. In fact, of the 361 tornadoes that reportedly touched down in Maryland in the 67 years that “official” records were kept, only 16 occurred between 11 p.m. and 5 a.m. Given the history, as well as the relatively lackluster atmospheric parameters in play, there was little concern for potential tornadoes with any storms on Sunday night.

Part of the reason tornadoes appeared unlikely was the lack of solar heating; to fire off a storm, the atmosphere requires CAPE — or convective available potential energy — to serve as the “fuel” of the thunderstorm. While we had a modest 1,000 to 1,300 joules per kilogram of CAPE, that seemed pale in comparison to the roughly 4,000 joules per kilogram typical in tornado-producing storms in the Midwest.

Accordingly, the forecast Sunday evening called for a chance of thunderstorms. The main concern was localized flooding from heavy downpours.

But should we have been worried? After all, a surprise event like this can always teach us things to be on the lookout for, and this particular case is no exception. Several localized features all overlapped in the right place at the right time to spin up the EF-2 tornado, which caused several million dollars in damage.

The storm develops

Just before midnight Sunday, a thunderstorm that unloaded torrential rainfall over Washington was intensifying as it marched eastward. The National Weather Service forecast office serving the Washington region issued severe thunderstorm warnings for central and eastern Anne Arundel County, including Annapolis, at 12:21 a.m. and 12:58 a.m. It also issued a Special Marine Warning at 12:04 a.m. for the Chesapeake Bay for a severe thunderstorm “capable of producing waterspouts,” effective until 2 a.m.

As these warnings were issued, the storm began to develop supercell characteristics. A supercell is a lone, discrete, rotating thunderstorm; the cell in question tracked along Route 50 across the Chesapeake Bay, apparently on a crash course with Kent Island, Md.

The Weather Service office in Mount Holly, N.J., which monitors Maryland’s Eastern Shore, issued a severe thunderstorm warning for this region at 1:05 a.m. and finally a tornado warning at 1:27 a.m.

Tornado ingredients gel

How did this ordinary thunderstorm turn into a destructive tornado producer?

The first localized ingredient was shear, a turning of the winds with height. When wind speed and direction change vertically in the atmosphere, storms embedded in an air mass can begin to twist and rotate. The below image shows the estimated wind shear at the time; notice the localized maximum reaching down the western coast of Maryland, where between 35 and 40 knots of shear were present. This is more than enough to start a storm spinning, and this probably contributed to the vigor of the supercell storm.


Wind shear present Sunday night in knots. (National Weather Service)

An additional ingredient that sometimes helps spawn tornadoes is surface boundaries — which separate contrasting zones of temperature. Any remaining fronts or boundaries can help to give storms a bit of a boost, while locally enhancing shear and further strengthening a storm. Given that the tornado touched down about 1:29 a.m., we can look at the 1 a.m. weather observations for clues.

In the below image, the yellow star marks the location of the Stevensville tornado. Notice that several stations south of the star indicate winds from the southwest, whereas many other stations across the upper Delmarva Peninsula and New Jersey feature a wind off the ocean from the east (the lines emanating from the circles indicate the direction the wind is coming from). It appears that a weak stationary surface front was established between sunset and midnight Sunday night in this vicinity, while the actual cold front itself lagged behind several hundred miles to the northwest.


Temperatures and wind direction before the tornado struck. (NWS)

Stationary fronts and warm fronts have a reputation for instigating spin-ups due to the shallow turbulence along the boundary. Indeed, we did have this type of front in place, which is always a red flag when it comes to severe weather forecasting. The Stevensville storm appears to have ridden along this boundary like rail cars on a track. At 11:54 p.m., an automated sensing station located at the U.S. Naval Academy in Annapolis reported a temperature of 82 degrees and a wind from the southeast at 9 mph. This abruptly shifted to a wind from the north an hour later. After the storm and front both passed at 1:54 a.m., the temperature had dropped to 74.

Taking a vertical slice through the storm and interpolating 16 layers of radar data, we can get a horizontal plot of the storm’s structure. When we do so, we can see that the main updraft of the storm, highlighted with bright colors, appears to be leaning to the right (northeast). This is a cause for alarm when it comes to thunderstorms, as it means that rain and precipitation will fall downwind of the updraft, thus not choking it out and allowing the storm to persist for an extended period of time. This type of organized structure often gives rise to tornadoes, as the rotating and leaning updraft can develop unencumbered.


(GR2Analyst)

One other key feature of boundaries like this is that they sometimes result in “locally backed flow.” In other words, they can enhance easterly surface winds just above the surface. This is clearly evident in the 8 p.m. weather balloon sounding just south of the area, launched from Wallops Island, Va., shown below.


Weather balloon sounding at 8 p.m. Sunday from Wallops Island, Va. (Data from National Weather Service)

The temperature at a given height is plotted in the black line on the right, whereas the left line charts dew point. To the right of the graph, wind barbs indicate the speed and direction of the wind at that level: a full tick mark corresponds to 10 mph, whereas a half tick is five. Notice that winds at ground level are weak and from due east, whereas they shift to the west with height. This is a classic signature of locally backed flow, and this “deep-layer shear” is exactly what spawned the tornado.

Radar reveals a beastly storm

As the storm developed in this environment, it quickly began to rotate over Annapolis. A four-panel radar shot (from the Weather Service radar at Dover Air Force Base in Delaware) is shown below.


Radar image showing storm over Annapolis late Sunday night. (GR2Analyst)

The top left frame is typical “reflectivity” and shows the intensity of precipitation; the curve on the southwest corner of the storm near Annapolis is known as a “hook echo” and is a classic signature of a supercell thunderstorm beginning to rapidly rotate.

In the second pane (top right), a velocity image is shown — green represents wind moving toward the radar, and red is away, so the combination of two strong colors juxtaposed right next to each other suggests strong rotation.

The third pane (bottom left) is a correlation coefficient image, and is not particularly useful yet.

However, the fourth shot (bottom right) plots storm heights, with the red representing storms exceeding over 50,000 feet in height! Clearly, this storm was, for lack of a better term, a beast!

As the storm crossed the bay toward the Eastern Shore, it rapidly intensified, with rotation becoming strong enough to prompt the 1:27 a.m. tornado warning, just a minute before the tornado moved onshore; previously, it had developed somewhere over the bay as a strong waterspout. Four minutes earlier, a buoy had reported a 68 mph wind gust one mile east of Eastport on the back of the storm, but the circulation had already passed.

Upon moving ashore in Stevensville, it became quickly apparent that a significant tornado was on the ground. The 1:28 a.m. radar shot is one of the most remarkable you’ll ever see in this part of the world. Below, there are four exceptional features.


A radar image showing a storm over Stevensville, Md., late Sunday night. (GR2Analyst)

Feature No. 1 is a reflectivity image, showing a spiral of precipitation being swirled around the storm’s rotating updraft and a resulting “doughnut hole” in the middle where the tornado’s vertical winds are so strong that rain is unable to fall.

Likewise, feature No. 2 shows how strong the contrasting winds are, indicating the power of the low-level rotation associated with the tornado underneath.

Feature No. 3 is a relatively sad and simultaneously staggering one: it is a “correlation coefficient” plot, and thus depicts how rough or jagged the shapes of raindrops, hailstones, and other atmospheric particles are. The blue clustering is an area of low correlation, meaning that there are unusual shapes in the atmosphere — in this case, tornadic debris, the remnants of people’s homes, vegetation, shrapnel and anything else picked up by the vortex. In the world of meteorology, this is referred to as a TDS — Tornado Debris Signature. It’s the worst possible type of signature one can see on radar.

Finally, the fourth plot shows the “overshooting top” of the storm, an area where the vigorous updraft has so much vertical momentum that it is able to puncture the tropopause (essentially the “ceiling” of the lower atmosphere), a remarkable feat for any storm!

This overshooting top is also visible in GOES-16 satellite imagery (below) as an area of exceptionally cool cloud tops plotted in gray.


A zoomed-in GOES-16 infrared satellite image shows a tornado-producing thunderstorm over the Chesapeake Bay on Monday. (NOAA)

After about five minutes on the ground, the 150-yard-wide tornado lifted and disintegrated near Route 18 at 1:33 a.m.

Chesapeake Bay waters helped fuel the beast

Part of what caught forecasters off guard with this storm was that it rapidly intensified over the waters of the Chesapeake Bay. However, this is not unusual for storms. Chris Kimble, a Weather Service meteorologist in Gray, Maine, recalls several occasions when he witnessed circulation within storms increasing dramatically over Sebago Lake.

“When a thunderstorm with weak rotation moves over a body of water such as a lake or bay, we have to pay close attention and be alert for the possibility of a tornado developing quickly,” Kimble said. “We think the smooth surface of the water, as compared to forested land, allows inflow to the thunderstorm to speed up,” fueling the storm. “This added inflow can help quickly spin up a tornado.”

Dan Satterfield, chief meteorologist at the CBS television affiliate in Salisbury, Md., said the warm waters over the bay may also have helped intensify the storm. “The waters are really warm now, over 86 degrees in some spots,” Satterfield said. “When the bay gets this hot, the storms often strengthen.”

Kimble also discussed the role of LCLs, lifting condensation levels, in spawning a tornado or waterspout. The LCL is where the base of a cloud will develop; the lower the LCL, the closer to the ground a storm can get, and the better the chance of a tornado managing to reach the surface. “The added moisture from the lake can assist an already-rotating updraft by causing the air to condense quickly,” Kimble said. This brings the cloud base closer to the ground, “causing a tornado or waterspout to appear.”

Visualizing the storm in three dimensions allows us to see the rotating structure of the cell quite well; as we look west toward Annapolis from Centreville, notice the updraft tower in the red spiraling upward into the storm. In the foreground, falling precipitation is dominant; Annapolis saw 1.72 inches of rain in one hour with the storm.


A three-dimensional view of the rotating structure of a tornadic storm over Maryland’s Eastern Shore on Monday. (GR2Analyst)

Likewise, we can plot the pillar of rotation responsible for generating the quick-hitting but destructive tornado.


Three dimensional view of rotating pillar of tornadic storm over Maryland’s eastern shore, July 24, 2017. (GR2Analyst)

Events like this always keep forecasters and meteorologists on their toes and serve as a reminder that we must never let our guard down. Always have a way to get notified of the latest watches, warnings and advisories from the Weather Service, because as Monday morning’s tornado showed, they very well may save your life.

Follow Matthew Cappucci on Twitter: @MatthewCappucci.