Up to 2 feet of snow could blanket most of Long Island, the National Weather Service said late Friday night in a signifcant upgrade of accumulation for the year’s first major storm.

The initial forecast of up to 18 inches, with the Twin Forks getting less, was revised when the nor’easter tracked more north instead of east, said meteorologist Joe Pollina at the service’s Upton office.

“It starts tapering off late Saturday night, and it’s done just around sunrise on Sunday,” Pollina said.

Flakes descended gently over Long Island late Friday night — a sort of calm before the fury of a storm loaded with the heavy kind of flakes and wind gusts of up to 55 mph, meteorologists said.

Long Islanders have been warned by public officials to hunker down at home for most of the weekend as a nor’easter brings flooding, whiteout conditions and damaging winds.

The brunt of the storm was expected to strike Saturday during daylight hours in a forecast that prompted officials to pull out lessons from superstorm Sandy and the 2013 snow dump that clogged and paralyzed the Long Island Expressway in Suffolk County for days.

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“Unless it’s an emergency, don’t be on the road,” Gov. Andrew M. Cuomo said at one of several news conferences held by public officials Friday. “It really is an unnecessary risk. Buy the groceries now. Do what you have to do now. But you need to stay home.”

Thousands of people and trucks, from the National Guard to Suffolk police in high-axle vehicles, waited in the wings to zoom to trouble spots, authorities said.

News 12 Long Island meteorologist Bruce Avery expected the snow should be steady by 5 a.m. and blizzard-like conditions will set in by 9 a.m.

But he had some good news: “The storm exits actually very rapidly early on Sunday morning, and there will be sunshine on Sunday to dig out from this.”

Though no one in New York called the nor’easter another “storm of the century,” they voiced worry over a mess with elements of Sandy and the February 2013 snowfall that dumped a surprise 33 inches, causing even plows to get stuck.

Suffolk County Executive Steve Bellone cited a dangerous combination — the heavy type of snow, high winds, amount of accumulation that was likely to cause coastal flooding, power outages and “blindout” conditions.

“This is a very serious storm that poses serious risks, and we are taking it very seriously,” Bellone said at a news conference by a storage facility holding some of the county’s 25,000 tons of salt.

In Nassau, County Executive Edward Mangano said more than 200 county employees and 100 snow plows would work through the storm.

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“We have the assets necessary to fight this storm,” he said.

Suffolk County public bus service and thousands of flights were canceled for Saturday, while the Long Island Rail Road and the Nassau County bus system said they would operate but evaluate service changes all day.

Suffolk Public Works Commissioner Gilbert Anderson said county officials will determine the status of Sunday bus service “as the storm develops and we see its impact on our roads and safety of the traveling public.”

At the Nassau Inter-County Express officials said they too expected the storm to make “travel challenging” and NICE bus safety teams would be out in the storm assessing conditions.

State officials have not ruled out barring nonessential travel on the roads Saturday, and Cuomo has said driving would endanger others, including people responding to emergencies and workers trying to clear the snow.

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PSEG Long Island is bracing for the problematic combination of high winds and heavy snow, which can weigh on trees and knock down power lines, said Jeff Weir, spokesman for the utility.

The company will have about 340 off-Island workers joining about 1,000 PSEG field workers and on-Island contractors to respond to any outages, he said.

“Personally, I’m more worried about the flooding than the snow” on Long Island, the governor said, adding that 600 members of the National Guard are ready to respond.

The U.S. Coast Guard, noting that seas could swell as high as 18 feet, urged boaters to seek safe harbor until the storm passes. A high surf advisory is in effect from 6 a.m. Saturday until 6 p.m. Sunday.