Accuweather.com said Thursday the storm "will evolve" into a blizzard in parts of the Northeast. The Weather Channel said peak snowfall for the Northeast will occur late Thursday night into very early Friday morning.
FlightAware.com, which keeps track of air traffic, said about 8,000 flights flying within, into or out of the United States had been delayed or canceled by Thursday evening.
Flights from Chicago east were affected as the Eastern United States prepared for a nor'easter bringing with it blizzard conditions.
The National Weather Service said the storm system "will raise havoc" this week as it spreads snow and sub-zero wind chills across parts of the region, CNN reported.
The nor'easter, forming off the East Coast, was expected to bring blizzard conditions by Thursday night to Long Island, N.Y., which is under a blizzard warning until Friday.
"Falling and blowing snow with strong winds and poor visibilities are likely," a NWS weather advisory said. "This will lead to whiteout conditions making travel extremely dangerous. Do not travel."
Authorities in New York said they may close the Long Island Expressway if conditions make driving highway too dangerous, CNN reported.
As of Wednesday night, New York was under a winter storm warning, as were many other densely populated areas in seven other states, CNN said. The Big Apple could get 9 inches of snow, sub-zero wind chills and turbulent winds, forecasters said.
"Deliver all of us from the impending snowstorm," New York Police Department chaplain Monsignor Robert Romano said in his invocation at Mayor Bill de Blasio's New Year's Day inauguration.
Albany, N.Y., could get 14 inches of snow, with wind chills of 15-to-25 below zero, NWS forecasters said.
Boston, which could get between 5 and 11 inches of snow by Friday, has declared a snow emergency.
Wind chills in parts of Connecticut are expected to range from 5 below zero to 20 below Thursday night and Friday, prompting the National Weather Service to issue winter storm warnings for most of the state through Friday morning.
Connecticut Gov. Dannel P. Malloy urged residents to allow extra time for commutes Thursday and Friday, CNN said.
"I am also asking that you avoid travel during the height of the storm Thursday night," Malloy said in a statement.
Dozens of school districts delayed or canceled classes ahead of the first day back after the winter holidays. Districts whose first day back is Friday, including Boston, also pre-emptively canceled Friday classes.
Mayors across the Northeast declared snow emergencies, banning parking on streets so crews could plow.
The Midwest won't be spared from bitter cold, either. In St. Paul, Minn., a pre-dawn temperature of 0 slowly slid to -10 by midmorning. The National Weather Service issued a wind chill advisory, saying wind chill values of between minus 25 and minus 30 were expected Thursday morning.
In Wisconsin's "frozen tundra," the Green Bay Packers will give a frosty welcome to the San Francisco 49ers Sunday for an NFC playoff game, when temperatures could bottom out at 17 below zero.
In Cincinnati, snow and rain are possible when the Bengals host the San Diego Chargers in another NFL match-up Sunday.