Baseball-size hail, damaging winds and at least one possible tornado raked through Virginia, Maryland and the District on Thursday in the region’s most intense severe thunderstorm day so far this year.

Five tornado warnings were issued in the D.C. region Thursday afternoon and night, beginning with storms west of Interstate 81 in Morgan and Berkeley counties in West Virginia. As the storms tracked southwest, tornado warnings were issued for parts of Clarke, Loudoun and Fauquier counties. The National Weather Service said it will be inspecting damage in Morgan County and that it is considering surveying reports of possible tornado damage in Berkeley and Clarke counties.

As of 11 a.m., none of the possible tornadoes had been confirmed. Photos taken during the storms do not actually indicate that any of the funnels were on the ground.

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Hailstones the size of baseballs fell in Middleburg, Va., on the border of Loudoun and Fauquier counties. Numerous cars were damaged, their windshields blown out and bodies dented.

One of our Capital Weather Gang community members, who goes by “Pamsm” on the blog, told us in a comment that leaves and small branches covered the ground in Middleburg after the storm, and it will be a “massive clean-up effort”:

I don’t know how widespread it was, but Middleburg got the hailstorm from hell (hellstorm?). Literally (and I use that term advisedly) golf-ball size hail that pounded down until the ground was covered. It sounded like an army of people throwing rocks at the house. It was terrifying. Many window screens destroyed (holes in screens, frames bent and broken), some storm panels shattered, roof tiles broken.
I am no spring chicken, but I have never seen anything like this before in my life. This is Kansas weather.

Wind damage was the most widespread impact of the storms. Dozens of trees fell across the region, including in the District.

This mother smartly advises that her kids get in the basement as branches from her walnut tree are ripped off by strong winds. Just because you’re not under a tornado warning doesn’t mean you can’t experience life-threatening conditions.

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Storm spotters reported over 150 instances of storm damage from hail and wind across Virginia and Maryland. There were no reports of injuries in the D.C. region, but one injury was reported from wind damage in Allegheny, Pa., when a tree fell on a car.

Thursday’s damage was widely distributed across the Mid-Atlantic. The vast majority of reports were damaging wind with a couple instances of large hail (2 inches or larger) and exceptionally high wind gusts (at least 75 mph).

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The first round of severe storms, which occurred in the late afternoon, were in what we call “discrete” mode — isolated, intense storms called supercells that have exceptionally strong, long-lived and rotating updrafts. In these updrafts, hailstones become suspended for a long duration, which means they can grow very large. And these updrafts vertically stretch and intensify small regions of low-level spin, leading to tornadoes.

One of the most dramatic supercells on Thursday occurred over the West Virginia panhandle, with a reported tornado over Berkeley Springs. The radar depiction of the storm is shown below. A red tornado warning broadly outlines the cell. A small arrow points to a hook-shaped appendage where the tornado most likely occurred. This counterclockwise-arcing hook echo is a common marker of the storm’s parent mesocyclone (rotating updraft).

The figure above shows the lowest radar elevation scan, closest to the ground. Dark reds and pinks denote very strong reflectivity (returned radar power), created by a mixture of torrential rain and hail. The figure below shows this same cell, but this is a map of the strongest reflectivity in the entire storm, which extended upward to an altitude of 50,000 feet.

The bright pink oval registers at 70 dBZ reflectivity — a value that is truly exceptional for the Mid-Atlantic region. Values this strong indicate the presence of giant hail.

There were three supercells that each produced damage in the D.C. region on Thursday — one in the West Virginia panhandle, another in western Loudoun County and the final in the Fredericksburg, Va., area.

Why did they form where they did? If you draw a line extending through the middle of each storm, that line would be from northwest to southeast. As the image below shows, these cells developed on the warm side of a stationary front that bisected our western suburbs, oriented from northwest to southeast. The warm side featured warm, moist winds from the south and an unstable air mass. Additionally, a deep layer of wind shear (increase in speed with altitude) was tracking across the region. The wind shear is what created the rotating updrafts, and thus the supercells.

Following the supercells, a bow-shaped squall line of continuous thunderstorms congealed along the Appalachians and rapidly surged eastward, striking the Washington region around 8 p.m. The storms were most intense south of the front over northern and central Virginia, once again due to the presence of a very unstable air mass. A solid wall of severe thunderstorm warnings was hoisted in advance of the squall line.

Individual thunderstorms within this line became locally intense, generating microbursts, which are exceptionally strong downdrafts creating explosive winds at the surface. In this “linear” mode of convection, downdraft winds tend to dominate and generate much of the destruction.

An example of a microburst, detected by Doppler radar, is show in the image below. Note the core of nearly 70 mph wind, close to the surface, west of Leesburg.

Why did a squall line evolve after the first round of storms?

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Unlike the supercells, the dynamic forcing for these storms was rooted in the middle atmosphere, courtesy of an approaching shortwave trough in the jet stream. The bowing line developed along the leading edge of a pocket of spinning air in the middle atmosphere, a region that generates strong upward motion over a large but narrow strip.

This relationship is illustrated below. It’s a busy image, but “X” marks the location of most intense, mid-level spin over Ohio. The pocket of counterclockwise spin (called vorticity) extends outward, shown by the large area of green shading. The bowing storm is located along the eastern edge of this advancing patch of vorticity, where vorticity values were rapidly ramping up.

Photos from the night

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