The first full day of winter brought a wild mix of weather across the United States, with ice and high wind in the upper Midwest and north-eastern New England states, flooding in the South and record-shattering temperatures along the mid-Atlantic region.
And freezing rain across much of eastern Canada turned roads and pavements into skating rinks and wreaked havoc on Christmas travel plans.
Snow and ice knocked out power to 400,000 homes and businesses in Michigan, upstate New York and northern New England, and also left more than 400,000 customers without electricity in eastern Canada.
"Thoughts are with those without power due to the ice storm," Canadian prime minister Stephen Harper tweeted. "Please stay safe."
At least nine deaths were blamed on the storm in the US, including five people killed in flooding in Kentucky, three traffic deaths in Oklahoma and a woman who died after a tornado with winds of 130mph struck in Arkansas. Four people were killed in Canadian highway crashes blamed on severe weather conditions.
In Toronto, warming centres were set up and the city shut down tram services and parts of the underground system. The city's giant Yorkdale Shopping Centre lost power.
Mayor Rob Ford called it one of the worst storms in Toronto's history. "My house is freezing cold, I have little kids, we might have to go to a hotel tonight, I'm not quite sure what we're going to do," he said. "It's not good to wake up and have a freezing cold shower."
Hydro Toronto said about 250,000 customers were without power as ice-coated tree branches snapped and brought down power lines. The utility's vice-president Blair Peberdy said crews were initially focusing on restoring power to two hospitals and a water treatment plant.
Anxious passengers found themselves stranded in airports from Toronto to St John's, Newfoundland. Canada's Via Rail advised commuters to expect delays on its routes between Toronto and Montreal or Ottawa, and police warned people to stay off the roads if possible. One Via Rail train got stuck in Oshawa because of downed power lines.
In the US, more than 700 flights had been cancelled and more than 11,000 delayed.
In Arkansas a woman was killed after a 130mph tornado struck in St Francis County. A man found in a field was taken to hospital in a serious condition, while the woman's three-year-old granddaughter and 25-year-old daughter were treated in hospital.
The icy weather is expected to make roads hazardous today from the upper Midwest to northern New England, just days before Christmas.
At the same time, high-temperature records for the date fell for the second straight day in the mid-Atlantic states because of a mass of hot, muggy air from the South.
In New York's Central Park, the mercury reached 70 degrees Fahrenheit (21 Celsius), easily eclipsing the previous high of 63F (17C) from 1998. Records were also set in Wilmington, Delaware, (67F, 19.5C), Atlantic City, New Jersey, (68F, 20C), and Philadelphia (67F). Washington tied its 1889 mark at 72 F (22C).
Temperatures are expected to return to normal by tonight.
The scene was much more seasonal in Vermont, where Lynne White of West Charleston listened to the cracking of falling tree branches and gazed at the coating of ice on her home.
"It's actually really pretty," she said. "Not safe, I'm sure, but it's pretty."
Heavy snow in Wisconsin forced dozens of churches to cancel Sunday services. Milwaukee got about 9ins and ice and snow in Oklahoma were blamed for three traffic deaths on slick roads.
In New York's St. Lawrence County, almost 2 inches (5 centimeters) of ice had accumulated by early Sunday, coating tree limbs and power lines, and a state of emergency was declared to keep the roads clear of motorists.
So far, the storm's impact appeared to fall well short of the havoc wreaked by the deadly ice storm that struck eastern Canada in 1998 when more than two dozen people died and about three million people - about 10% of Canada's population - was without power during four days of intermittent freezing rain.
Marie-Eve Giguere, a meteorologist with Environment Canada, said the 1998 storm involved far more freezing rain and ice accumulation than over the past few days.
In Canada, the severe weather conditions, which saw people skating down streets in Kingston, Ontario, were suspected to have played a role in three fatal crashes in Quebec and another in Ontario over the weekend.
In Kentucky, five people were killed in flooding caused by the storm system. The bodies of three people were pulled from the Rolling Fork River after their vehicle was swept away, a fourth person drowned in Carroll County after a four-wheel drive vehicle overturned in high water, and a body was discovered in Ballard County near a car abandoned in a flooded ditch.