Winter storm warning for Philly. Forecasters see up to 13 inches, 45 mph winds, Sunday into early Tuesday

The National Weather Service has hoisted a winter storm warning for the entire region from 1 p.m. Sunday through 10 a.m. Tuesday, Groundhog Day, with the potential for a foot or more of snow.

Winds gusting to 45 mph are likely to cause significant blowing and drifting, and coastal flooding is possible.

Whether the result will be a “classic” nor’easter with the biggest snow in five winters or a classic virtual misfire by the computer models, be grateful you haven’t been a floor at a local supermarket this weekend.

With evermore confidence, meteorologists have been saying that an unusually long-duration winter storm is forecast to begin affecting the region Sunday and linger into early Tuesday

The weather service sees rain mixing in at the Shore, cutting down on snow totals, with some mixing possible for a time inland Sunday night and Monday morning; however the precipitation eventually will become all snow all the way to the beach towns.

Caprice would be a primary concern for the accumulation forecast, Chad Shafer, a lead meteorologist at the weather service office in Mount Holly, said Saturday. Amounts could vary widely. Some “banding” — heavy snow falling in narrow corridors, while intensities would be reduced nearby — is expected.

That said, the weather service sees a 90% chance of at least 6 inches for Philadelphia, and significant snow is all but inevitable.

“The cat’s out of the bag,” said Tom Kines, senior meteorologist with AccuWeather Inc.

As folks continue stocking up on milk, eggs, bread, butter, and assorted beverages, don’t be surprised to see tweaks in accumulation forecasts.

» READ MORE: Snow is snubbing Philly in January, and February is looking mild across the nation

But anyone who has spent more than a few winters around here is aware that snow has been known not to happen.

And chaos evidently has had a far better season than the computer models that attempt to calculate the next moves of the atmosphere.

For all the advances in forecasting and computer technology, in the 21st century the I-95 corridor has experienced some historic snow-forecast busts.

Veteran meteorologists have experience with watching the wheels fall off a forecast.

One of the most infamous busts occurred 20 years ago in March 2000, and coincidentally it involved a storm that was to begin affecting the region on a Sunday and linger into Tuesday. The drumbeat for a cosmic snowfall that would shut down the I-95 corridor gained volume on a Wednesday and grew only louder as the week progressed.

On Friday, the usually staid National Weather Service warned of the potential for a storm of “historic proportions,” with accumulations of one to two feet or more possible.

The region ended up with about one to two inches of what was largely an unidentifiable frozen substance.

In January 2015, what appeared to be a megastorm was targeting the I-95 population center. Again, it was to commence on a Sunday in Philadelphia. To the north, New York Mayor Bill de Blasio took the extraordinary step of clearing the city’s streets of all traffic, cutting off coveted food deliveries. “This will most likely be one of the largest blizzards in the history of New York City,” he said.

» READ MORE: 30.7 inches fell upon Philly 25 years ago, and snow hasn’t been the same since

New York got a generic five inches. Philadelphia, with up to two feet in the forecast, ended up with 1.2 inches.

It was dubbed “The Flizzard of 2015.” Gary Szatkowski, then the head of the local weather service office, apologized.

His boss, Louis W. Uccellini, said that wasn’t necessary. The forecasts were based on the best science available.

Computer models are remarkable tools and meteorologists consult scores of them run by the United States, the United Kingdom, Canada, and elsewhere. They share at least one trait with their human interpreters: fallibility.

From the get-go, they have to rely on incomplete and imperfect observations, which are used to capture the three-dimensional “initial condition” of the atmosphere. Applying the laws of physics, they then undertake massive calculations to measure how the weather systems have changed in the previous six hours and how it might change in the next six, on out in time.

What explains the observation issues? Most of the planet is uninhabited. About 70% of the Earth’s surface is water, and lesser-developed countries have priorities other than sending up weather balloons.

The gaps are so significant that forecasters run “ensembles” — multiple models tweaked to account for what’s missing and possible errors — to come up with likely outcomes. Any errors worsen with time, and the machines continue to have issues, meteorologists say.

“The models have been struggling as of late,” said Judah Cohen, a scientist with Atmospheric and Environmental Research in Massachusetts.

Regardless of what does or doesn’t happen, local supermarket registers are likely to accumulate some cash.

“In January and February grocers count on snow to drive customers back into stores, balancing out the dip in sales volume following the holidays,” said Jere Downs, a former executive for Kroger’s, the nation’s biggest chain. “This is really important.”

People, evidently, are prone to overbuy before the storm, and The Inquirer has found no evidence of widespread milk binge-drinking.