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Yet another coastal storm due; 'too close for comfort,' says weather service

A hard to predict coastal storm is brewing: Forecast for Philly is anywhere from a miss to accumulating snow.

A man cleans the snow off of his car on the 800 block of Walnut Street during a winter storm, in Philadelphia, Wednesday, March 7, 2018.
A man cleans the snow off of his car on the 800 block of Walnut Street during a winter storm, in Philadelphia, Wednesday, March 7, 2018.Read moreJESSICA GRIFFIN / Staff Photographer

With that pockmarked meringue of snow and ice holding its own against the March sun, computer models are confident that yet another coastal storm will take shape Sunday into Monday.

As of Saturday morning  – and for this particular threat that is a huge caveat – the odds are more favorable for snow south of and east of the city.

But the National Weather Service in Mount Holly sees a 10 percent shot of a 6-inch snowfall in Philadelphia, and the numerical forecast models suggested a 100 percent chance of uncertainty.

"I don't think the models are cohesive here," wrote Walt Drag, a lead forecaster.

The weather service has cautioned that forecast confidence was "low," calling the storm "too close for comfort." It noted that the forecast models have tended to bring storms farther north in their later runs.

This particular threat has a number of moving parts, and a key piece of energy from the Pacific did not arrive until Friday morning, said Evan Duffey, a meteorologist with AccuWeather Inc.

It has a long way to pass through land-based observation network. Weather-balloon data is critical for the calculations by the various computer models.

"Some storms, we have more confidence than others," said Duffey. "This is one of those situations, there's just not enough agreement."

Duffey details might not come into focus until Saturday night – if then.

As we learned with the last two assaults, details can be elusive even as a storm is maturing.

Computer models suffer from a fundamental problem: The atmosphere, extending from the Earth's surface to the boundaries of space, is an ever-restless subjectt.

Observation gaps abound,  which makes it impossible to predict precisely how it is going to behave.

"It's an imperfect science," said Duffey. "We can't measure every molecule."

In the end, numerical weather prediction is a prodigous math problem, and nature gets to correct the papers. Rarely does it give a perfect score.

And as Gary Szatkowski, former chief meteorologist at the NWS regional office in Mount Holly, said on Twitter about the potential of a storm: "Worth watching but wouldn't be adjusting any plans at this point in time."