From the ABC 7 Weather team

Nor’easter to cap off D.C.'s October weather; here comes winter

October 28, 2011 - 02:03 PM
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This year, Mother Nature is not wasting any time with the snow. A Nor'easter will be moving up the coast this weekend, even before Halloween.

Nor'easters can blanket major cities with heavy snow in the winter, producing the most accumulation out of any type of East Coast storm. This year, Mother Nature is not wasting any time. A Nor'easter will be moving up the coast this weekend, even before Halloween.

Let’s first talk about how these storms form. A powerful jet stream roars into the West Coast or screams southward from western Canada and eventually meets up with the southern jet stream that moves west to east across the U.S.’s lower tier. The northern part typically reaches the southern jet stream (a process called phasing of the jet streams) and forms an explosive low along the Gulf Coast or in the Southeast. The upper-level flow pattern becomes elongated south to north or “deepens,” allowing the surface low to move northeast up the East Coast.

With the northern branch transporting cold air south and the counterclockwise circulation around the low pressure streaming in moisture from the Atlantic, a swath of snow forms just to the north and west of the surface low’s track. The track of the low is crucial in identifying where the rain/snow line will be located and where the heaviest swath of accumulation will fall.

snow

Average annual snowfall across the Washington, D.C. region. Photo courtesy of the National Weather Service, Sterling, Va.

In this weekend’s storm, the northern branch of the jet stream is dominating the flow and the surface low looks to form just to the southeast of the Metro area and slide northeast, quickly deepening or becoming stronger. This will allow the precipitation duration to be a bit shorter than a typical Nor’easter that really gets going along the Gulf Coast and explodes off the North Carolina or Mid-Atlantic coast.

In October, we look for temperatures to be well below freezing through the vertical profile of the atmosphere to generate snow. In addition, it can be difficult to get snow accumulation unless the snow falls at night or comes down heavy enough to overcome the warmer surface temperatures. However, the problem comes in with leaves still on the trees at this time of the year. The heavy weight of the snow on the leaves can weigh down tree branches and cause tree damage.

Is it uncommon for a Nor’easter to open up our eyes this early to winter? In October, yes, but not in early November. In the Baltimore/D.C. region, the earliest Nor’easter in recorded history was Nov. 6 to 7, 1953, when the earliest recorded major snowfall (4 inches or more) hit Baltimore with 5.9 inches. The famous Veteran’s Day Storm occurred Nov. 11, 1987. A total of 11.5 inches blanketed Reagan National Airport with 13 inches in Prince George’s County. This storm stranded cars on the Capitol Beltway.

Has snow ever occurred before Halloween in the District? Yes, on Oct. 10, 1979, the day of the World Series. That storm marked the earliest measurable snowfall in both Baltimore and D.C., with 0.3 inches. Trace amounts in Baltimore also fell on Oct. 9, 1895, and 1903, and in Washington on Oct. 5, 1892.

Stay tuned to ABC7 and WTOP for the latest developments with the potential weekend storm.

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