A winter storm is bearing down on the Northeast. Where the most snow may fall.

A significant winter storm was bearing down on the Northeast early Friday, set to deliver hefty accumulations of snow and ice to portions of the Appalachians and New England — with potentially hazardous conditions coinciding with post-holiday travel. Up to 8 inches of heavy, wet snow is expected — with a jackpot zone near New York City.

Winter storm warnings are in effect in the Lower Hudson Valley, including New York City, as well as in northern New Jersey, Long Island and southwest Connecticut. Isolated totals could approach 10 inches, especially if any stagnant snow bands develop Friday evening. Snowfall rates could exceed an inch per hour at times, with an outside chance for thundersnow.

In western Pennsylvania, meanwhile, hazardous ice accretions are likely.

The wintry wallop is unfavorably timed, coinciding with the peak of post-Christmas travel and, for those working on Friday, the evening commute. Major highways, including Interstates 95, 81 and 80, will be affected.

“If you must travel, keep an extra flashlight, food, and water in your vehicle in case of an emergency,” wrote the National Weather Service. “Check local Department of Transportation information services for the latest road conditions.”

Ice storm warnings, meanwhile, are in effect in western and west central Pennsylvania, the Maryland Panhandle and extreme western New York state. Localized ice accretions over a quarter inch are likely. Freezing rain will wash away any pretreatment on the roadways, and a perilous glaze will quickly coat any exposed surface.

“Power outages and tree damage are likely due to the ice,” warned the National Weather Service in Pittsburgh. “Travel could be nearly impossible. The hazardous conditions will impact the Friday post-holiday travel.”

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Travel issues so far

On Friday, the day after Christmas, FlightAware data showed more than 1,100 cancellations within, into or out of the United States, and more than 3,800 flights were delayed.

Travelers can consult the website’s Misery Map for a visual of which airports are most affected.

By midday, New York airports were leading in cancellations, with John F. Kennedy International reporting 133 flights canceled, or 20% of the airport’s total flights, followed by Newark Liberty (96 cancellations, or 16%) and LaGuardia (73 cancellations, or 17%).

JetBlue faced the most total cancellations, with 225 flights (or 22% of the carrier’s total flights), followed by Delta Air Lines with 185 (or five percent of its total).

On Thursday, there were 607 delays and 3,752 delays.

Major airlines — including JetBlue, American, Delta, United and Spirit — have issued travel alerts for the Northeast due to severe weather and are waiving change fees and fare differences within specific dates.

Travelers should check their airline’s website or app for the most up-to-date information on their flights.

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Conditions driving the storms

The nasty weather is a result of a low-pressure system pushing east from the Midwest. Ahead of it, chilly high pressure has entrenched cold air at the surface. Warmth and moisture are riding up and over that shallow lip of subfreezing air hugging the ground.

With warm air a mile above the ground, rain is able to fall. But the subfreezing surface means it transforms into ice once it hits the ground.

In the icing zone, it’ll be about 41 degrees at 5,000 feet altitude, but only 27 or 28 degrees at the surface. That’s why icing will be such a big hazard in Pennsylvania.

There’s even a chance of thunder-ice, which occurs when thunder and lightning accompany freezing rain. There isn’t any instability, or thunderstorm fuel, at the surface, but there is some elevated storm fuel a mile or so above the ground.

In New York City, meanwhile, the forecast is for heavy, wet snow. That’s where it’s colder at all levels of the atmosphere, so snowflakes can easily form and survive down to the ground. Snow should arrive around 4 p.m. Snowfall rates will become moderate to heavy at times between 7 p.m. and 11 p.m., which is when the bulk of the accumulation will occur. Isolated thundersnow is also possible.

The heaviest snow will fall just northeast of a band of frontogenesis — or a zone of changing temperatures at the mid levels of the atmosphere. That frontogenesis zone creates a density ramp of sorts that air pockets can ride up to climb higher into the atmosphere.

But what goes up must come down, so those air pockets release their moisture, which falls as heavy, wet snow.

That’s why there’s an increasing risk that the highest levels of precipitation may be near or just northeast of New York City, or perhaps into the southwest corner of Connecticut. In the event some snow banding parks over the same area for several hours — a narrow area could see 10 inches of accumulations.

Most of southern New England east of Interstate 84, however, will see minor accumulations.