Long Island and New York City were bracing for a weekend storm that could blanket the area with 6 to 12 inches of snow, forecasters said.

A blizzard watch was in effect from Saturday morning through Sunday afternoon, with winds of 25 to 35 mph that could topple power lines and trees, the National Weather Service said.

Snowfall amounts were not yet pinned down, as the Island is on the northern edge of the heaviest snow line, and even a slight shift in the track to the north or south “could make a huge difference,” said Bill Korbel, News 12 Long Island meteorologist.

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Coastal flooding also is likely, the weather service said.

Large, breaking waves could lead to dunes erosion and some wash over on barrier islands, including Fire Island.

“Lives may be at risk for people who put themselves in harm’s way,” forecasters said.

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Officials urged New Yorkers to stay home, if possible.

Gov. Andrew M. Cuomo, Mayor Bill de Blasio, the Nassau and Suffolk executives, the Metropolitan Transportation Authority and PSEG all said they were undertaking abundant preparations — and urged residents to do the same.

“This storm could have a significant impact in communities throughout the downstate area — so I am directing all relevant state agencies to be on alert and ready to respond as the weather develops,” Cuomo said Thursday in a statement. “I encourage all New Yorkers in the region to plan ahead, avoid unnecessary travel, and above all — stay safe.”

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Late Thursday, the National Weather Service reduced its snowfall prediction for Long Island.

MTA chairman and chief executive Thomas Prendergast said in a statement train service would be provided “as long as it’s safe to do so.”

The Long Island Rail Road “may modify or suspend service in heavy snowfall, during ice storms and blizzards, or if sustained winds over 39 mph occur, especially if there are frozen switches or there is a loss of third rail power,” according to Cuomo’s office.

PSEG Long Island said Thursday that it was setting up for a storm with heavy snow and high wind gusts, which can bring down trees and wires. An unspecified number of additional crews will be at work, including tree trimmers. Workers will perform system checks on vital transmission and distribution lines, ensuring there are enough materials, fuel and other supplies. Customers who lose power can call 800-490-0075.

Many airlines that fly out of Kennedy, LaGuardia, Newark Liberty International and Long Island MacArthur said they were relaxing their ticket cancellation and change policies and canceling some flights in advance.

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That list included JetBlue, American Airlines, Southwest, United and Delta.

Around the country, Southwest led Friday’s list of pre-canceled flights with 401, followed by American at 317, according to the flightaware.com website.

In Lindenhurst, where superstorm Sandy repairs are not complete, the risk of flooding spurred Billy Gasparini, 43, to move his cars north to his mother’s house.

“Especially with a damaged bulkhead down the street, I wouldn’t be surprised if we saw a few feet of water,” said Gasparini, whose house was inundated during Sandy. “There’s not much you can do but get your stuff to higher ground and pray for the best.”

Other Lindenhurst residents, including Vincent Bentivegna, 35, stocked up on food and rock salt at the supermarket. Bentivegna recently broke his leg, and plans to stay in this weekend and hire some kids to shovel his driveway.

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Nassau County Executive Edward Mangano said Friday’s storm preparations include spreading a salt mixture to prevent ice buildup.

Nassau will deploy more than 200 employees, around 100 pieces of snow removers, such as plows and spreaders, 28,000 tons of road salt and 3,600 tons of sand.

And there should be fewer equipment problems as its average age now is 8 years old, down from 21 years old several years ago.

“That’s good news, as they will be prepared to plow our roads and not break down, as we’ve seen in the past,” Mangano said.

Suffolk County Executive Steve Bellone said the county has “been in communication with towns and villages regarding emergency preparedness operations.”

Suffolk police say they plan to dispatch 14 Humvees — surplus vehicles from the military — to aid stranded motorists and to respond to emergency calls.

New York City is issuing a hazardous travel advisory, effective Saturday and Sunday, which means officials recommend not driving those days.

The city is deploying 579 salt spreaders on Friday and 1,650 plows when at least 2 inches of snow accumulates, officials said.