An intense thunderstorm tore across Northern Virginia and Southern Maryland on Monday, unleashing bursts of damaging winds and hail. Some of the hail was large — almost as big as we ever see in the Washington region.
That appears to be an apt description based on an inspection of photos from witnesses.
So much hail poured down that it coated the ground like snow in Linkwood, Md., about five miles east of Cambridge. The stones were so big that they damaged homes, puncturing siding and peeling off paint. They also dented cars and shattered mirrors.
“My parents live just outside of Cambridge,” Caroline Stutts, who follows Capital Weather Gang on Facebook, wrote. “Their car windows were busted out and gutters destroyed.”
What caused these enormous spiky hail stones to develop?
The responsible storm cell, shown in a radar snapshot below, was very large and intense. It developed over high terrain in Northern Virginia and quickly grew in size and strength as it tracked east, producing hail near Manassas, south Alexandria and Waldorf. By the time it reached Calvert County, it was really humming, after having triggered severe thunderstorm warnings. In Huntingtown, the hail was up to two inches across, bigger than golf balls.
Embedded in this storm was a core of exceptionally high-radar reflectivity values. Reflectivity refers to the amount of radar energy that has been backscattered by precipitation targets, including raindrops and hail stones. Values of 50 to 55 dBZ are typical of torrential rain; those as large as 70 dBZ (shown in the figure) almost always indicate the presence of large hail.
The storm cell was a prolific hail producer, with stones of many sizes. It may seem paradoxical that the largest pieces of ice that fall out of clouds occur only on the warmest days, not the dead of winter. But to grow large, hail stones must be levitated for long periods high in the storm cloud, to accrete layer upon layer, and this requires a vigorous cloud updraft.
Buoyant energy arises from a very warm and humid air mass close to the ground, and this stokes a powerful updraft within the storm cloud. Several hours of sunshine after morning showers enabled temperatures to climb into the low 80s. The amount of available buoyant energy quickly increased.
The severe storm cell that tracked from near Warrenton, Va., to Cambridge was a lone wolf, in that it was an isolated entity with an unlimited command on available buoyant energy.
Additionally, the wind shear (increase in wind speed with altitude) was exceptionally strong for mid-May, thanks to an intensifying jet stream disturbance aloft. Wind shear of this intensity causes the updraft to lean downwind, allowing it to unload its cache of rainwater and hail over long distances. In such an environment, buoyancy forces can act unimpeded and permit the updraft to accelerate to high velocity. Higher velocities enable the updraft to suspend larger stones aloft.
The hail spikes probably formed as large, embryonic stones descended rapidly through a mass of smaller stones; the small stones accreted onto the outside of the large stones until they became too heavy for the updraft to suspend. Then they crashed to the ground.
The hail that ordinarily accompanies storms in the Washington area is around the size of a pea. Once in a great while, the stones are as big as a golf ball. Instances of anything bigger than that are rare. Some of the biggest hail in the region occurred during the La Plata tornado 20 years ago, when stones reached the size of softballs (4½ inches in diameter).