Updated at 3:07 a.m. ET: A crippling and potentially historic winter storm barreled toward the Northeast on Thursday, threatening tens of millions of people with 2 feet of snow. Boston canceled school and braced for one of its worst blizzards of all time.

Airlines encouraged fliers to change their plans and get out of the way. There were already delays of more than two hours at O’Hare International Airport in Chicago, where tangles can snarl air traffic across the country. More than 2500 flights had been cancelled by early Friday, according to flightstats.com.

The culprits were a so-called clipper system moving through the Upper Midwest and a low-pressure system headed for the waters off New England. When they converge, probably late Friday, they are expected to sock the region with its heaviest snow in at least two years, and perhaps much longer.

“When this hits, it’s going to come down very hard,” said Tom Niziol, a meteorologist for The Weather Channel. “This is something we haven’t seen in a while, particularly in New England.”

The National Weather Service put the New York City area and Long Island under a blizzard warning and said those areas could get more than a foot of snow. Earlier in the day, the weather service warned that travel in Massachusetts, Connecticut and Rhode Island could become nearly impossible.

Full coverage from The Weather Channel

Forecasts called for as much as 9 inches of snow across central Michigan, a foot and a half in the Hudson Valley region of New York, and 2 feet or more across coastal New England. Possible hurricane-force winds off Massachusetts and Rhode Island also made flooding a threat.

In Boston, the storm had the potential to take out century-old records. The city’s biggest snowstorms since 1892 were a 27.5-inch blast in February 2003 and a 27.1-inch dumping exactly 35 years ago, in 1978. Mayor Thomas Menino closed city schools for Friday and pleaded for common sense.

“Stay off the streets of our city,” he said. “Basically, stay home.”

Light to moderate snow is expected to spread through the Great Lakes on Thursday and could reach as far east as parts of New England and New York City by Thursday night, according to forecasters for The Weather Channel.

Snow should begin Friday in Boston and Hartford, Conn., and grow heavy at times during the day in New York, New England and parts of Pennsylvania, the forecasters said.

The most intense part of the storm was expected to hit Friday night and Saturday, with as much as 3 inches of snow falling per hour in coastal New England, including Boston, Hartford and Portland, Maine.

By Saturday evening, snow should taper off in Boston and the storm is forecast to pull off the coast of Maine by Sunday morning, The Weather Channel said.

RELATED: Detailed storm timeline from The Weather Channel

In New York, Mayor Michael Bloomberg said the city was readying plows and said crews would work extended shifts.

“It’s been a quiet winter, but we knew that February could be a tough one,” said the city’s sanitation commissioner, John Doherty.

For at least some people there, the storm was a chance to profit.

“Shoveling, cleaning cars, anything you need me to do,” Isaac Morales told NBC affiliate WHDH in Boston. “I already have rock salt. I already have shovels. I’ve got extra bodies. I’ve got everything so I’m all set.”

But for survivors of Hurricane Sandy, including thousands of people still displaced and many more with disrupted lives, it was more serious. A much smaller snowstorm followed Sandy in late October.

“People were just miserable, unhappy, and it started to get cold,” Annie Petraro of Long Island told NBC New York. “Things just weren’t good. And now it’s freezing, it’s gonna snow.”

The Long Island Power Authority, which was strongly criticized for a slow response to the hurricane, said that it was planning for this one and making sure it had enough people working and enough supplies.

More than 130 flights into and out of O’Hare were canceled Thursday, and more than 70 were already canceled for Friday, according to FlightAware.com. More than 400 flights into and out of Newark Liberty International Airport were canceled for Friday, as were 100 for Boston Logan.

American, Delta, United and other major airlines said they would waive their fees to change flights, which can run to $150, for people going through major airports in the Northeast, including Logan in Boston and LaGuardia and Kennedy in New York.

Amtrak canceled some runs of its Downeaster train line, which runs from Brunswick, Maine, south to Boston.

RELATED: Travelers brace for ‘monster storm’

Ski resorts were excited by the prospect of a major snowstorm.

“It is perfect timing because it will just remind everybody that it is winter, it’s real, and get out and enjoy it,” Tom Meyers, marketing director for Wachusett Mountain Ski Area in Massachusetts, told The Associated Press.