Quick-moving snow blindsides some, misses others

A snow burst quickly reduces visibility in Riverdale, Md. on Route 410 around 2:30 p.m. Tuesday. (WTOP/Dave Dildine)

Dave Dildine, wtop.com

WASHINGTON – A few localized spots in the Washington area experienced a brief but intense bout with winter weather Tuesday. Although many gawked at the occasional flurries seen throughout the midday hours, others found themselves cornered by swift-moving heavy snow showers or “snow squalls.”

From where did these ferocious bursts of snow come?

The skies over Washington during the mid-morning hours Tuesday featured innocuous, puffy clouds. Many of these cumulus clouds swelled into shapes resembling the heads of cauliflower by late morning. Despite the weak sun angle numerous breaks in the sky cover allowed for the air near the ground to heat up – the high at Reagan National Airport was a toasty 34 degrees.

More importantly, however, the air just above the ground was significantly colder due to an upper level disturbance (picture a big blob of cold air the size of Ohio sloshing overhead). The great contrast between the temperatures at the ground and the extremely cold temperatures aloft created instability in the atmosphere. Additionally, moisture from the Great Lakes teamed up with the cold air over the area.

“Very strong northwest winds throughout the low levels of the atmosphere sent air from the Great Lakes southeastward into the Washington Area,” Stephen Konarik, meteorologist for the National Weather Service in Sterling, Va. tells WTOP.

“The atmosphere over our area was unstable but it needed the moisture source from the Great Lakes,” he explains. The puffy clouds of noon became energized, swelled, and soon dark streaks under their bases could be seen regionally from any tall vantage. The cells functioned like height-challenged thunderheads, drawing lift from the surrounding atmosphere. The precipitation underneath these “cells” became moderate to heavy in intensity.

Perhaps the most vigorous snow shower came screaming southeastward from the northern Blue Ridge Mountains around 1 p.m. It skirted south of Frederick then barreled its way southbound along I-270. It impinged upon northern Montgomery County by 1:30 p.m. Daylight faded and winds howled wildly as the menacing cloud approached Germantown and Montgomery Village. Seemingly within the blink of an eye, visibility dropped to less than a quarter of a mile and a thin coating of white stuff was deposited on nearly every exposed surface including most roads. This led to numerous accidents along the Interstate 270 corridor as reported by WTOP’s Bob Marbourg.

“It was the exposed and elevated concrete road surfaces that may have been our undoing early this afternoon,” Marbourg said, noting that black asphalt can retain more heat which can allow the snow to melt on an otherwise sunny day, whereas, the more reflective concrete bridge decks were likely the most accident-prone areas during Tuesday’s episode.

ABC 7’s Live Super Doppler Radar showed the storm blasting through the Rockville, Kensington, Silver Spring and Wheaton areas between 1:55 p.m. and 2:10 p.m. Some surprised residents reported unofficial observations of 1/4 inch of snow in less than 10 minutes. By 2:20 p.m. the snow burst had already raced into Hyattsville and Riverdale and had begun to weaken.

Motorists in northwestern Virginia had their winter weather driving skills tested by 6:30 p.m. as a more linear band of moderate to heavy snow snaked its way southeastward from the mountains near Winchester into northern Fauquier County. This thin snow squall lashed at Route 50, Route 7 and Interstate 81, challenging drivers with a narrow zone of windswept, accumulating snow that led to numerous traffic backups.

The daytime snow bursts were as robust as they were fleeting, In their wake they left a trace to up to a couple tenths of an inch of snow, enough to coat most surfaces including roads. Yet the merits of the phenomenon weren’t based on accumulations but in the suddenness of the snowfall itself. The prompt reemergence of the sun following the passage of these showers quickly did away with the minor accumulations. After sunset, isolated areas in northwestern Virginia saw over an inch of snow accumulation.

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