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 may have spawned a tornado near Fredericksburg.
Support our journalism. Subscribe today. arrow-right
The National Weather Service received more than 25 reports of damaging winds and large hail in Stafford and Spotsylvania counties in Virginia, as well as the city of Fredericksburg, as a violent, rotating 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. A funnel cloud was viewed by Weather Service storm spotter about four miles south-southeast of Fredericksburg.
The Weather Service is surveying damage from the storm Friday to determine if a tornado touched down. It is also examining damage in connection with a possible tornado in Howard County.
Advertisement
Story continues below advertisement
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.
‘Particularly dangerous’ tornadoes slammed Pennsylvania and New Jersey on Thursday
The rotating storm near Fredericksburg was one of four to the transit the Washington-Baltimore region on. 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. A supercell tracked along Interstate 70 in Howard County before dipping into northern Anne Arundel County, and brought down a few trees. It prompted tornado warnings west of Ellicott City and east of Columbia.
Inside the Fredericksburg storm
Of the supercells in the Washington-Baltimore region, the Fredericksburg storm was the most intense. 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.
Advertisement
Story continues below advertisement
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).
Advertisement
Story continues below advertisement
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).
Story continues below advertisement
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.