Thursday marked the third time this week severe storms charged through the Washington region. This time, several of the storms were spinning, including one that spawned a weak tornado near Columbia, Md., and another that unleashed 90 mph winds near Fredericksburg, Va.

The tornado near Columbia was rated an EF-0 on the 0 to 5 scale for tornado damage. It was on the ground for just one minute a little after 5 p.m. Thursday, reaching a maximum width of 50 yards as it tore across the Snowden Square shopping mall.

“Damage was limited to a quarter-mile path, and peak wind gusts were estimated to have been around 70 mph,” the National Weather Service wrote in its storm survey.

The Weather Service found tornado damage at a BJ’s store and tire center where “a section of the wall facade, estimated about 50 feet by 20 feet, was removed,” the storm survey stated. “An employee at the tire center noted that nearby shopping carts were lofted and then rapidly blown to the north.”

To the south, the Weather Service determined that despite a large number of damage reports near Fredericksburg, a tornado had not touched down. It ruled straight-line winds were responsible for a swath of damage in western and southern Stafford County. The winds peaked near 90 mph, stronger than those unleashed by the Columbia tornado.

The Weather Service received more than 25 reports of damaging winds and large hail in Stafford and Spotsylvania counties, as well as the city of Fredericksburg, as the violent storm swept across the area. The hail was as a big as billiard balls (2.25 inches across) in Stafford County.

Most of the wind damage was in the form of downed trees, several landing on homes.

“Stafford County Emergency Management reported a total of eleven houses considered affected from tree damage and that one house was determined to be destroyed around Truslow Road and Melvin Drive,” the Weather Service storm survey reported.

While the damage was attributed to straight-line winds, the storm was indeed rotating. A funnel cloud was viewed by Weather Service storm spotter about four miles south-southeast of Fredericksburg.

Thursday’s severe storms came amid an outbreak in the Mid-Atlantic and into the Northeast that produced several tornadoes in Pennsylvania and New Jersey. The storms erupted in warm unstable air ahead of a strong cold front sweeping toward the coast, while a strong high-altitude disturbance surfing the jet stream supplied additional energy.

The rotating storm near Fredericksburg was one of four to transit the Washington-Baltimore region Thursday. Three supercell thunderstorms passed through Washington’s northern suburbs, two prompting tornado warnings:

  • A supercell tracked from eastern Montgomery County into Prince George’s and Calvert counties, producing isolated tree damage, but did not require a tornado warning.
  • A supercell produced scattered tree damage near Westminster, Md., where a tornado warning was issued. No tornado was confirmed here.
  • A supercell tracked along Interstate 70 in Howard County before dipping into northern Anne Arundel County, and brought down a few trees. A wind gust of 62 mph was recorded at BWI Marshall Airport and the storm prompted tornado warnings west of Ellicott City and east of Columbia, where a tornado was confirmed (as described above).

Inside the Fredericksburg storm

While the Howard County supercell produced a tornado, the Fredericksburg storm was the most intense of the four that impacted the region. It was detected and monitored by Weather Service Doppler radar in Sterling, Va. A panel of three radar views from around 5:30 p.m. is shown below.

The leftmost panel shows radar reflectivity, which measures the intensity of precipitation, at the lowest scan level of the radar. Fredericksburg and Falmouth are located in the bottom center of the image. One noteworthy feature is a prominent appendage or “hook echo” in the southwestern region of the storm. The hook is generated by heavy rain wrapping counterclockwise around a low- to mid-level updraft circulation, called the mesocyclone.

The mesocyclone is not a tornado, but the parent circulation from which a tornado can develop.

The middle panel shows the same view, but a scan called “composite reflectivity.” This composite is built from the highest energy return values measured at all levels of the storm, not just the lowest scan. It shows a large region of extremely high reflectivity (purple zone) which was likely concentrated in low- to mid-level layers of the supercell, just on the edge of the updraft. Such tremendously high values almost always indicate large hail, in the context of a supercell.

The last image shows the Doppler velocity, or the speed and direction of winds associated with the storm. Airflow toward the radar is shown in green, and moving away from the radar as red. There was a concentrated region of red adjacent to green (shown by the white arrows) immediately northwest of Falmouth and Fredericksburg, the hallmark of storm rotation.

Other features to note in these images include lightning strikes (white lightning symbols), severe thunderstorm warning polygon (yellow lines) and tornado warning polygon (red lines).

What ingredients conspired to create multiple supercell storms across the Interstate 95 corridor? An unstable air mass and humid air mass, for one, which provided the buoyant energy for strong cloud updrafts.

More critical to rotating storms, however, was a very strong wind shear (increase in wind speed with altitude) — exceptionally high values (45 to 50 mph of shear over the lowest few miles of atmosphere) for midsummer in the Mid-Atlantic.

Some of the shear came from strong westerly winds in mid levels as a disturbance in the jet stream (shortwave trough) moved out of the Great Lakes. Additional enhancement of these winds likely arrived in the form of a mid-level vortex embedded in the larger flow, called a mesoscale convective vortex (MCV).

The MCV was thought to have been triggered by upstream (early morning) thunderstorm complexes over Ohio. Meteorologists were tracking this feature through the morning and afternoon, and anticipated that it would interact with the increasingly unstable air mass east of the Blue Ridge.

The MCV also generated a strong belt of rapidly rising air as it approached I-95, focusing and intensifying development of the band of supercell storms.