Blizzard Alert:Northeast Snowstorm could be among the worst of alltime

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 ChannelForecasts 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.The snow is expected to pick up early Friday afternoon and by Saturday at 8 a.m. blizzard conditions will be in full force along several major cities in the Northeast, from New York City to Portland. The Weather Channel's Jim Cantore reports."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 ChannelIn 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."