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Michael Dwyer, Associated Press
Irv Rosenberg, of Boston, uses cross country skis on the Esplanade in Boston, Saturday, Jan. 24, 2015. A winter storm warning covering Boston and Hartford, Connecticut was in effect through 7 p.m. as the National Weather Service said to expect 4 to 8 inches of wet snow to fall by the time the storm moves out.

NEW YORK — Mayor Bill de Blasio warned New Yorkers on Sunday to prepare for what could be the biggest snowstorm in the city's history — with a potential accumulation of as much as 3 feet.

"This could be a storm the likes of which we have never seen before," the mayor told a news conference in a Manhattan sanitation garage where workers were preparing plows and salt for the massive cleanup.

They face a challenge: Snow on about 6,000 miles of city roadways — double the distance between New York and Los Angeles.

Gov. Andrew Cuomo urged commuters to stay home or leave work early on Monday. He said there's a possibility that major roadways, including the Thruway, I-84 and the Long Island Expressway, and public transportation, such as bus, subway and train service, could be shut down before the evening rush hour.

The National Weather Service issued a blizzard warning Sunday for the New York and Boston areas starting Monday night and into Tuesday. Forecasters say the storm could drop 2 to 3 feet of snow from northern New Jersey to southern Connecticut, accompanied by up to 65-per-hour wind gusts and low visibility.

The mayor said that Monday morning should be fairly normal, and schools would be open, though after-school activities were scrapped.

He urged people to return home as early as possible in the afternoon — to ease what de Blasio called the "huge problem" anticipated for the evening rush as heavy snow starts falling, and to keep streets clear for emergency crews.

The work will be divided into 12-hour shifts, with 2,400 sanitation employees on each one. Nearly 500 salt spreaders will be out ahead of the snowfall, then 1,500 plows when 2 inches of snow is on the ground.

Major highways, hospitals, schools and hilly areas will be targeted first.

Alternate side parking has been suspended for Monday, as has garbage collection. Con Edison is at the ready to repair any power lines felled by the wind, resulting in blackouts.

Despite all the emergency measures, "assume conditions will be unsafe," the mayor said. "Yes, there will be delays in everything."

At New York's LaGuardia Airport, about 25 percent of Monday's departing flights were canceled already Sunday, and about 7 percent at Kennedy International Airport.

The mayor was joined by Sanitation Commissioner Kathryn Garcia, Transportation Commissioner Polly Trottenberg, and the chief of the Office of Emergency Management, Joseph Esposito.

De Blasio held up a piece of paper showing the city's 10 worst snowstorms and said this one could land at the top of a list that goes back to 1872, when 21 inches was recorded.

"My message to all New Yorkers is, prepare for something worse than we have seen before; now is the time to get ready for this extreme weather," he said, noting that it could surpass the 2006 storm when 26.9 inches of snow fell on New York City.

But at Manhattan's Penn Station, Cicero Goncalves was ready for the best: snowboarding in Vermont in mounds of fresh, soft white stuff.

First, he had to get there. And already on Sunday, travel from New York City was tricky.

Dressed for the storm in a fake grizzly-bear coat with a hood that looked like the animal's head, the 34-year-old flight attendant from Queens waited for an Amtrak train to Vermont because he expected the flight he and a friend had hoped to take would be canceled.

"We'll get there before it snows, and we're coming back when the storm is over, on Thursday," he said with a broad smile. "So we should be able to get some fresh snow, some good powder."

New Yorkers staying in the city also went into action, with preparations large and small.

A Manhattan Home Depot store sold about twice as many shovels over the weekend as it normally does.

Transit officials hoping to keep the subways running smoothly planned to use modified cars loaded with de-icing fluid to spray the third rail that powers trains.

Emergency crews were out Sunday filling more than 1,000 potholes to ease snow removal, Trottenberg said. Department of Transportation employees also will make sure bridges are passable.

City buses will be equipped with snow tires or chains if necessary.

And any homeless person who shows up at a shelter will automatically be welcomed. In addition, shelters will be opened to others who are left stranded.