With a blizzard watch in effect on Long Island and in New York City, state and local officials are urging residents to stay off the roads on Saturday — and take it slow if you have to be out and about.

“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,” Gov. Andrew M. 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.”

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

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

Nassau County Executive Edward Mangano said Thursday morning that the county is fully prepared for the storm, touting supplies that have yet to be used this winter and an equipment fleet that is far less worn than in years past.

“The good news is that this is mostly a Saturday event,” Mangano said, “when there’s less of a traffic problem.”

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Flanked by police, fire and public works officials, Mangano ran down a list of Nassau’s storm response measures at a news conference outside Office of Emergency Management headquarters in Bethpage.

“We’re well-stocked,” he said. “This season we really haven’t had to use any of our supply.”

The county, Mangano said, has more than 200 employees who will respond to the storm this weekend, using roughly 100 pieces of snow removal equipment, such as plows and spreaders, 28,000 tons of road salt and 3,600 tons of sand.

The equipment is, on average, 8 years old, Mangano said, 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.

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Employees will begin spreading a salt mixture on county roads Friday to prevent ice buildup, Mangano said.

Also Friday, the county will likely activate its nonemergency hot line to allow for calls on issues such as downed trees, and keep 911 free for life-or-death matters.

Mangano also advised residents to stay off the roads, citing the probability of high winds and low visibility. He warned of coastal flooding as a “major concern,” advising that residents in typical flood zones take extra precautions.

“We are preparing for the worst, hoping for the best,” he said.

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

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Bellone said snow removal equipment is at the ready, and county officials “will continue to monitor the storm closely” and also communicate with the National Weather Service and state emergency management officials as the storm develops.

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

Three 5-ton truck heavy-duty vehicles are at the ready to use in extreme conditions should that be necessary, a police spokeswoman said Thursday.

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.

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The National Weather Service said Thursday morning that nor’easter-like conditions are expected to hit Long Island starting Saturday morning.

The blizzard watch, which runs from 6 a.m. Saturday to 1 p.m. Sunday, means there’s potential for 8 to 12 inches of snow, northeast winds of 25 to 35 mph, with gusts up to 50 mph, along with temperatures in the lower 30s.

The blizzard watch also means risky driving conditions, with visibility at one-quarter mile or less because of blowing snow.

PSEG Long Island said Thursday that it was preparing for a storm with heavy snow and high wind gusts, which can bring down trees and wires. The utility said it was bringing in an unspecified number of additional crews over the weekend, including tree trimmers, and was performing system checks on critical transmission and distribution lines and making sure it had enough materials, fuel and other supplies.

“PSEG Long Island continues to prepare for the approaching storm,” said LIPA spokesman Jeff Weir. “In addition to doing all of the systems and logistics checks and tree trimming, we are pulling in all available crews for Saturday and Sunday.”

PSEG vice president John O’Connell noted that while snow and wind “normally don’t pose a serious problem to the electric system, icing on lines and trees can increase the possibility of downed wires and power outages.”

Customers who lose power can call 800-490-0075.

With Mark Harrington, David M. Schwartz and Alfonso A. Castillo