On Friday afternoon, a lone supercell thunderstorm dropped impressively large hail, up to golf ball size, over northern Washington and close-in Maryland suburbs, including the cities of Silver Spring and Takoma Park. In some areas, so much hail fell that it covered the ground like snow and lingered over an hour. (See more pictures and video at the bottom of this post,)

Earlier that day, the atmosphere didn’t provide many clues that forewarned this very isolated and intense storm, and forecast models painted a generally benign afternoon for most. But with analytical hindsight and some impressive radar imagery, we can briefly recap this event.

Among the storm’s most impressive attributes was its radar presentation, shown below. The huge mass of red, embedded purple and a telltale hook is a dead giveaway for a vigorous, rotating thunderstorm known as a supercell. With a hook so well defined, one immediately worries about a tornado on the ground. But fortunately, this was not to be; the supercell instead would instead take on the identity of a “hailer.”

Radar depiction of a supercell about to move into northern Washington on Friday. The red polygon is a severe thunderstorm warning issued by the National Weather Service in Sterling, Va. (Weathertap.com)

Why the lone cell and none other (in the immediate D.C. vicinity)? Instability (the tendency of heated air to spontaneously overturn or “convect”) was limited. Overcast, fog and showers through the morning curtailed early heating of the ground. At about noon, the overcast finally broke, but just two to three hours of warming in the strong late-April Sun was sufficient to generate a weakly unstable atmosphere — ranking about a one out of a scale of three in terms of total energy available for storms (three being the highest).

But there was a surprising amount of wind shear through a deep layer — that is, an increase in wind speed with altitude. When shear exceeds about 45 mph, one begins thinking about rotational tendencies in thunderstorms, although this usually doesn’t happen on a widespread basis until the shear gets into the 55 mph range. Nevertheless, the 45 to 50 mph shear in place (stronger than forecast that morning) was enough to develop some mid-level rotation in the storm cloud.

The intensity of the shear, which causes storm cells to tilt or lean over downstream, is suggested by the dramatic three-dimensional radar image of the supercell (provided by Ian Livingston), shown below. Looking more like a windblown plume of smoke, the red surface outlines the precipitating core of the storm. The strongest updrafts, areas of vigorously rising air, probably pushed upward to 40,000 feet. Purple colors in the interior denote a large mass of hailstones.

A three-dimensional depiction of the supercell’s precipitating core. Purple cores indicate hail aloft. (Ian Livingston)

The storm cell developed along a convergence zone (location where low-level air streams flow together) that was part of a weak front draped along the Chesapeake Bay, Washington and northeastern Maryland. Convergence creates a focused uplift of air. Cooler air lay to the north and east of the boundary, and the Washington region had only started to break out into the narrow “warm sector” ahead of an approaching cold front (located over West Virginia). I was able to identify the general location of this boundary using observations of wind direction, instability and temperature, as shown on the map below. Note the location of the lone supercell (arrow) within this frontal zone.

Surface analysis showing frontal boundaries (cold front, warm front) and the location of the isolated supercell (purple arrow). (Adapted from the National Weather Service Storm Prediction Center)

Any time a strong thunderstorm cell approaches and interacts with a frontal boundary, interesting things can happen — namely, the storm updraft can acquire spin in the low levels. This probably helped the preexisting mid-level mesocyclone of the supercell build downward toward the surface, creating the hook echo shown above.

Why the supercell did not become tornadic is uncertain, but tornado formation is a delicate process, in fact quite rare compared with the total number of annual thunderstorms. Nearly two-thirds of all supercells fail to develop tornadoes.

It’s worth mentioning that for as ominous as this storm appeared on radar — and the shock and awe of large hail, torrential rain and a few damaging wind gusts — very few people in our region experienced this level of weather Friday afternoon. For most, this was not a “totally missed” forecast. The extremely isolated nature of this storm underscores the difficulty of predicting convective storms in general. The perspective is this: A supercell passing over anyone’s home in our region, on an annual basis, is a very unlikely event, even on days that auger for more widespread severe storms.

Below see some dramatic photos and video of the storm structure, hail and rainbows in its wake…

Storm structure

The hail

Hail in Bethesda, April 21, 2017. (Helen Hocknell)