Dangerously cold arctic air swept across a huge swath of the United States making travel treacherous, forcing schools to close and prompting officials to plead with residents to stay indoors. A shift in a weather pattern known as the "polar vortex" triggered a drastic drop in temperatures to lows not seen in two decades, and coincided with wind chill warnings in much of the east of the country. Comertown, Montana, recorded the lowest wind chill value so far at -63 Fahrenheit (-53 Celsius) while North Dakota, South Dakota and Minnesota were not much warmer. That was significantly colder than the South Pole, which recorded a wind chill reading of -29 Fahrenheit. Even the typically temperate Deep South was feeling the chill with a hard freeze warning threatening crops and livestock. On the East Coast, a brief lull in the deep freeze greeted morning commuters, but rainstorms threatened to turn roads and sidewalks into ice rinks overnight as temperatures were expected to plunge. Deaths blamed on the frigid weather include a worker crushed by a 100-foot (30-meter) pile of salt being prepared to treat roads on Friday, and a 71-year-old woman suffering from Alzheimer's disease who froze to death after getting lost in New York state. The body of a 90-year-old woman was found face down in the snow next to her car in Ohio on Monday morning, the Toledo Blade reported. At least a dozen other people were reportedly killed in crashes on icy roads, including four people whose sport utility vehicle slid off a rural Minnesota highway and fell into the Mississippi River. Four Chicago men aged 48 to 63 died of apparent heart attacks while shoveling the snow over the weekend, the Chicago Tribune reported. The cold snap came after two massive winter storms snarled travel, grounded thousands of flights and dumped as much as two feet (60 centimeters) of snow in the first few days of the year. More than 4,300 US flights were canceled Monday -- nearly half of those in Chicago -- and more than 6,500 were delayed, according to FlightAware. The city of Chicago was among scores of municipalities which told parents to keep their children at home rather than risk sending them out into winds so bitter that skin could freeze in a matter of minutes. The governor of Minnesota canceled school across the entire state on Monday. Illinois Governor Pat Quinn declared a state of emergency and called up the National Guard to help rescue stranded motorists as high winds whipped up blinding snow. We are facing a dangerous combination of low temperatures, black ice and snow drifts," Quinn said, as he urged people to stay off the roads. Quinn praised the "heroic" efforts of National Guard troops who cleared a 375-vehicle backup and a forestry officer who rescued seven stranded people and two of their pets using a snowmobile. In Milwaukee, Wisconsin, officials warned residents to stay indoors and urged schools to shut down as temperatures dropped to -12 Fahrenheit, with the wind chill making it feel like -37. "Police are reaching out to homeless citizens in order to help them find the nearest shelter," city spokeswoman Sarah DeRoo told AFP. Indianapolis mayor Greg Ballard barred everyone except emergency workers from driving at the height of the storm Sunday and urged residents to stay indoors unless absolutely necessary on Monday. "This extreme cold poses a serious health and safety risk," he warned. But with thousands of people without power after electrical lines were felled, home was not always the best option. Those who couldn't stay with family or friends were urged to seek out community centers which were opened as temporary shelters. The extreme cold disrupted flights and classes in Canada as well. In the Atlantic island province of Newfoundland, more than 30,000 people were without power. Nationwide, officials warned of "treacherous" travel conditions, but meteorologists said a warming trend would begin mid-week.