This story was updated based on storm survey results from the National Weather Service which confirmed tornadoes in Ranson, W. Va., in eastern Frederick County, Md., and in Virginia’s Northern Neck.

For about four hours Monday evening, over a path of about 80 miles from northeast West Virginia and across northern Maryland, tornado warnings blared. They sounded as a long-track supercell or rotating thunderstorm dropped tornadoes while leaving behind several pockets of damage.

Another tornado damaged homes in Virginia’s Northern Neck on Monday afternoon.

The evening storm that tracked north of Washington and Baltimore, notable for its longevity, produced three areas of damage. In northeast West Virginia, a twister rated EF1 on the 0 to 5 scale for tornado intensity, downed trees and wires and damaged structures in Ranson, which is just north of Charles Town.

In its report on the tornado, the National Weather Service said the twister was on the ground for five minutes between 6:27 and 6:32 p.m., covering 1.8 miles. It was as wide as 125 yards while unleashing winds up to 90 mph. The tornado damaged several structures, including a warehouse, where one person was injured.

The storm then unleashed a second tornado east of Frederick, Md., near Libertytown, where weather radar indicated debris lofted into the air and there were new reports of trees and wires down. The Weather Service confirmed Wednesday that an EF1 twister touched down at 8:14 p.m. and was on the ground for 0.8 miles, or about two minutes, before lifting. Its peak winds were 90 mph and it was up to 200 yards wide. The tornado damaged more than 100 trees but avoided any structures.

Additional storm damage, mostly to trees, was reported in Carroll County, Md., near and just southeast of Westminster. It’s less clear whether that damage was due to a tornado or just straight-line thunderstorm winds.

The tornado in Charles Town and twister in Frederick County were part of a small but long-lived complex of storms tracking to the northeast Monday evening.

How the storm developed and evolved

Shown in the panel below is a supercell thunderstorm with a pronounced hook echo (soon to be tornado-warned), at 5:30 p.m., tracking out of far Northern Virginia, into the West Virginia panhandle. The same storm complex continued to trigger tornado warnings over the next four hours, the one shown here at 9:30 p.m., north of Baltimore.

The storm complex was “cycling” over a four-hour period. That is, it was triggering a sequence of tornado warnings, based on Doppler radar indications of a rotating region in the lower cloud, called a mesocyclone.

What would cause behavior such as this? The day featured a slowly destabilizing atmosphere, as a warm front tracked slowly through the D.C.-Baltimore region. But the degree of air mass instability was on the low end, not vigorous enough to get meteorologists unduly concerned for severe thunderstorm activity.

The wind shear — or increase in winds with altitude — was significant. We had at least 45 to 50 mph of wind speed increase between the ground and middle levels of the atmosphere. If a thunderstorm cell was able to get going, then those strong wind changes could impart a spin to the updraft.

Although Monday featured strong shear, the instability was marginal. Accordingly, the Weather Service’s Storm Prediction Center declared just a 1 out of 5, or “marginal,” storm threat, and the primary damage mode was anticipated to be straight-line wind. Sometimes, however, tornadic thunderstorms can erupt in marginal setups, particularly when warm fronts are involved.

Let’s take a closer look at that warm front, shown below and circled, because you might otherwise have a hard time finding it on the surface weather map, around the time that the supercell storm developed.

The warm front was in a weakening state as it migrated north. But it featured several key things — acting in a marginal manner — that were enough to get rotating storms fired up. These include:

  1. Focused ascent of increasingly warm, humid air streaming toward the north.
  2. Subtle shifts in wind speed and direction in low levels, leading to a zone causing the air to spin horizontally (termed helicity).
  3. A corridor along which the storm complex would be able to propagate to the northeast over a four-hour period.

Earlier in the day, the same warm front triggered another tornado in Virginia’s Northern Neck, which was rated EF2 by the Weather Service. It had peak winds of 120 mph and was on the ground for 5 miles.

There was no tornado warning for the twister, which caused damage to homes in Northumberland County, across the Potomac River from southernmost St. Mary’s County in Maryland.

Monday’s atmospheric environment was not a dead ringer for rotating thunderstorms. It was a day on the margin, in many ways, and a reminder that subtle ingredients can sometimes combine in just the right measure to create some dangerous outcomes.