NEW YORK - Millions of people in the United States hunkered down Sunday in anticipation of brutal weather from a dangerous Arctic blast that forecasters warn could send freezing temperatures plummeting to record lows.The northeast of the country and parts of Canada have been in the grip of crippling heavy snow and deadly sub-zero temperatures since the turn of the year and the deep freeze is now ripping through the US Midwest and threatening usually warmer areas further south.The wind chill from the rare "polar vortex" could make it feel as cold as -55 Fahrenheit (-48 Celsius) in places, weather forecasters say, prompting authorities in several towns and cities to issue warnings telling people to stay indoors and even stock up on food.In such cold conditions, exposed skin would suffer frostbite in as little as five minutes, experts have cautioned. In New York, which declared a state of emergency when storm Hercules swept in on Thursday, John F. Kennedy Airport ceased operations for more than two hours because of freezing rain and snow after a Delta Airlines jet from Toronto slid into a snowbank. None of the 35 people on board were hurt, ABC News said, but at least a dozen people have died in the cold conditions since the turn of the year and travel has been badly disrupted, with thousands of flights canceled or delayed, ensuring a miserable end to the holiday season for some."Winter Storm Ion is spreading a swath of heavy snow across the Midwest, and its icy tentacles will also bring wintry weather into the south and parts of the east. Following closely behind Ion will be a blast of brutally cold air," said The Weather Channel in its Sunday morning forecast.The Midwestern states of Minnesota, where Governor Mark Dayton has already announced schools will be closed Monday "to protect all our children from the dangerously cold temperatures," and North Dakota were expected to bear the brunt of the worst weather.Chicago, Detroit and St Louis all saw more snowfall overnight, while Sunday's National Football League playoff showdown between the Green Bay Packers and the San Francisco 49ers at the open-air Lambeau Field in Wisconsin looked set to be one of the coldest NFL games in history. The Packers say they will help fans battle the big freeze by handing out free coffee, hot chocolate and hand warmers, while supporters would also be allowed to bring blankets and sleeping bags. Added to the mix was freezing rain forecast to hit the south and east, affecting New York and Washington, with the extreme weather expected to continue into the early part of the week in many places across the country. Among the deaths blamed on the weather was a worker killed on Friday when he was crushed by a 100-foot (30-meter) pile of salt being prepared to treat roads in the Philadelphia area, media reports said. A 71-year-old woman suffering from Alzheimer's disease froze to death after walking out into the cold and getting lost in northern New York state, according to authorities.