Snow, perhaps more than a foot, is all but certain this weekend for Philly

Based on what the computers and their human interpreters are saying, a key question this weekend will be whether measuring the snow in the Philly region will require a ruler or a yardstick.

This no doubt will be a moving target, but on Friday morning, the National Weather Service in Mount Holly was seeing eight to 14 inches for Philly, said meteorologist Alex Staarmann. Several inches were possible even at the Jersey Shore.

For Philly, that would be the first double-digit snowfall in 10 years.

A wild card would be a potentially unpleasant atmospheric parfait that would add ice to the mix on Sunday, and computer models Friday were suggesting that mixing was likely near I-95 and in Delaware and South Jersey. However, the weather service expects that to yield to all snow Sunday night.

While this is all quite a complicated meteorological setup, in essence Arctic air is pressing southward and it is going to interact with an impressively juicy storm to the south.

“Having the Arctic front come through before the onset of wintry precipitation, that’s really concerning,” said Ray Kruzdlo, the staff hydrologist in the weather service office, where “it’s all hands on deck.”

Below-zero windchills are expected Saturday morning, prompting a cold-weather advisory, and temperatures in Philly may stay below freezing the rest of the month.

The timing and duration of precipitation aren’t among the strong suits of computer models.

The weather service’s winter storm watch, which covers the entire region, all of Delaware, and most of Pennsylvania and New Jersey, is in effect from 7 p.m. Saturday until 1 p.m. Monday.

The daytime Saturday “looks fine if you have to get out,” said Tom Kines, senior meteorologist with AccuWeather Inc.

The weather service is listing the likeliest starting time as the early morning hours of Sunday, with snow likely into the early morning hours of Monday.

Sunday is going to be one of the colder days of the winter with temperatures in the teens and lower 20s. The weather service introduces the possibility of freezing rain and sleet by 1 p.m., with a forecast temperature of 19 degrees.

» READ MORE: The promise, peril, and whimsy of a potential double-digit snowfall in Philly

Yes, it can rain when it’s below 20 degrees at the surface, and precipitation doesn’t get much more dangerous.

Snow and sleet, liquid that freezes on the way down, can at least provide traction on the roads. Rain that freezes on contact becomes an ice sheet. Also, when freezing rain accumulates on fallen snow it can bring down trees and power lines.

Peco has heard the storm rumors (who hasn’t?) and will have crews on call through the weekend, said spokesperson Candace Womack.

The threat of ice is related to the possibility of warm layers of air, borne on onshore winds from the ocean, at levels of the atmosphere where precipitation is formed.

That could well happen Sunday as the coastal storm intensifies, said Kruzdlo, and winds build from the Northeast, perhaps gusting past 20 mph. Any rain or sleet would encounter very cold air at the surface, locked and dammed in place by the Appalachian Mountains.

“That’s the complexity of living where we are so close to the ocean,” Kruzdlo said. “We have tens of thousands of observations at the surface,” he added, but data from the upper atmosphere is wanting, adding challenges to forecasting changeovers.

Along the I-95 corridor, storms of purely snow are the exceptions, Kruzdlo said.

In the chess matches between science and the nonlinear chaos of the atmosphere, chaos has been known to win.

One of the more notable busts occurred in January 2015 when forecasts called for an I-95 East Coast snowstorm so ferocious that the mayor of New York imposed a curfew.

Philly was supposed to get a foot or more, and ended up with an inch or two. That prompted the head of the Mount Holly weather service office to issue a public apology.

His boss at the time, weather service head Louis Uccellini, said no apology was necessary: Science has its limits. Busts have been known to happen in the battle of science against nonlinear.

This time around, meteorologists are all but certain something “impactful” is going to happen.

Said Kruzdlo, the slim chance of this storm “not being significant is leaving us.”