Storms, forecasts, moving fast

A resident of Federal Street in South Philadelphia cleaning off her car Wednesday. Expect some snow over the weekend. (MICHAEL BRYANT / Staff Photographer)

The official forecast for Saturday took a precipitously whiter turn during the day today.

The National Weather Service snow map posted Friday morning showed less than inch for most of the region, and on the afternoon map totals jumped to 2-4 inches for the outer suburbs.

Recall that last Sunday, Tuesday’s was to be a nuisance event; instead, Jan. 22 turned out to be 10th snowiest day officially in Philadelphia since recordkeeping began in 1884.

In a pattern such as this, snow threats can pop up in a hurry, said Evan Myers, meteorologist and chief operating officer at AccuWeather Inc.

Alberta Clipper systems, so named because they originate in western Canada, have been riding the perimeter of the formerly obscure, now nationally celebrated, polar vortex.

Typically, the systems show up here fairly dry, but not always.

On occasion – as happened Tuesday – they can affect the Philadelphia region significantly when they pass to the south and mine Atlantic moisture.

The center of Saturday’s system will pass north of the region, but the attached front is expected to set off a period of snow around here and might even tap moisture from the Chesapeake Bay, said weather service meteorologist Walter Drag.

“Tomorrow is going to be a wild day,” said Myers, adding that the region might even see squalls at some point.

Another system is due to pass to the north Sunday night, but shouldn't have much impact here.

Something more significant is possible later in the week, Myers said.

As for the following weekend, AccuWeather’s forecast for Groundhog Day, which also happens to be Super Bowl Sunday, calls for a chance of snow showers in North Jersey, which is hosting the game.

But expect that Feb. 2 outlook to change daily. For the cold and volatility are going to continue. “The overall pattern isn’t going to change,” Myers said.

And, yes, it’s been awhile since we had a winter like this one.

Said Myers, "It is the most sustained cold pattern we’ve seen in 20 years,” he said.

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