The National Weather Service has issued a blizzard warning for Suffolk County for Thursday, calling for 10 to 14 inches of snow. The warning is in effect from 3 a.m. to 6 p.m.

A winter storm warning remains in effect from midnight to 6 p.m. in Nassau County, where forecasters are calling for about 10 to 11 inches of snow.

The fast-forming storm calls for a mix of rain and snow from 3 a.m. to 6 a.m., followed by moderate to heavy snow until about 3 p.m., said meteorologist John Murray of the National Weather Service in Upton. Starting about 5 p.m., the snow will start tapering off west to east, with skies clearing for a dry night, he said.

Gov. Andrew M. Cuomo urged New Yorkers to “avoid all unnecessary travel” during the storm. Nassau County Executive Edward Mangano advised residents to stay off the roads “if possible.”

The center of the low pressure system was forming over Virginia just after midnight and was expected to clear Cape Cod in Massachusetts some time Thursday afternoon, according to one prediction from the weather service.

Temperatures will likely fall into the 20s over the course of the day, feeling colder with the wind chill.

advertisement | advertise on newsday

“Conditions will rapidly deteriorate around daybreak, impacting the morning commute,” the weather service said in a tweet.

Several Long Island school districts have already announced they will be closed Thursday.

In New York City, Mayor Bill de Blasio said all public schools will be closed Thursday because of the storm.

“We do not want people getting stuck on roads,” said Brian Ciemnecki, a meteorologist for the weather service in Upton, because snowfall rates could make traveling “nearly impossible tomorrow morning.”

During the morning commute, expect conditions to be “rather treacherous, as the snow will come down moderate to heavy at times during this time frame,” the weather service said. It recommended that residents avoid travel but said those who must drive should have an emergency kit in the car.

As cold air sweeps in, there could be an inch or two of snow just east of New York City by 6 a.m., the NWS said.

For a brief period, snow could fall at a rapid rate — two to three inches an hour on Thursday — before tapering by midafternoon. Heavy bands of snow could also reduce visibility at times to a half to a quarter of a mile or less, the service said.

Officially, the National Weather Service defines a blizzard as a storm that contains large amounts of snow or blowing snow, with winds in excess of 35 mph and visibilities of less than a quarter-mile for an extended period of time — at least three hours.

advertisement | advertise on newsday

North winds of 20 to 30 mph, with gusts up to about 50 mph, are predicted in Suffolk, the weather service said.

Winds from 15 to 25 mph, with gusts up to 35 mph, are predicted in Nassau, forecasters said.

In the run-up to the storm, the Long Island Rail Road was deploying extra staff and specialized equipment, including 360 snowblowers, up to 35 pickup-truck plows and seven rail-bound jets, Cuomo’s office said in a news release.

Antifreeze trains will be deployed throughout the systems to spray de-icer on the third rail to prevent ice buildup, and nonpassenger patrol trains will work to prevent snowdrifts from forming on the tracks, according to the release.

Extra personnel will pre-salt station platforms before the storm and clear platforms of snow during and after the storm, while waiting rooms will be kept open, the governor’s office said.

advertisement | advertise on newsday

Mangano said the Nassau Department of Public Works was out pre-treating roadways.

“Nassau County is helping to ensure residents have a safe commute by brining main county roadways, bridges and overpasses to prevent black ice,” Mangano said in a news release issued Wednesday morning. “DPW is at the ready to plow and salt county roadways.”

Suffolk County spokeswoman Vanessa Baird-Streeter said in an email Wednesday night: The county and its police department “have taken the necessary steps to ensure that Suffolk County is prepared for the impending forecast. Our Emergency Operations Center will open at 5 a.m. and will be staffed with emergency response personnel for the duration of the storm.”

In Hempstead Town, snow crews were making sure plows were ready to go out in the storm and were preparing to mix up brine and clear storm drains.

The Town of Huntington, meanwhile, canceled all town events for Thursday, along with garbage collection.

Out east, Riverhead Supervisor Sean Walter said a winter storm emergency would likely be declared in his town Thursday to facilitate snow removal on roadways. All vehicles would be required to be parked off the local roads, and Riverhead police would be authorized to impound parked or abandoned vehicles that affect roadway cleanup.

With Lisa Irizarry