A winter storm pummeled southern Ontario Thursday, causing hundreds of accidents on highways and roads and flight cancellations and delays on one of the busiest travel days of the year.

Airport officials at Toronto's Pearson International Airport said more than 200 flights had been affected as the combination of heavy snow and freezing rain threw a wrench into a day that was already expected to see a 30 per cent increase in passenger traffic for the holidays.

Many flights were two to three hours behind schedule.

Thursday morning commute in Windsor.

There were also long lineups at Ottawa International Airport, and delays at other airports from Halifax to Vancouver, as the knock-on effect of disruptions to the schedule in Toronto took its toll.

Passengers were advised to call or go online to check on their flights, and to allow more time to get to the airport because ground traffic was moving very slowly.

Environment Canada said 10 centimetres of snow fell on southern Ontario overnight Wednesday and another 10 centimetres by Thursday afternoon.

Jump start in frigid Winnipeg

Some areas of southern Ontario were expecting to receive up to 40 centimetres of snow by the end of the day Thursday.

More than 400 traffic accidents in Ontario

Snow plows were out in force across southern Ontario, but struggled to clear roads and highways. Atrocious driving conditions on the nation's busiest shopping day were also exacerbated by poor visibility in some places.

By mid-morning, the Ontario Provincial Police had already reported more than 400 traffic accidents, with at least two people seriously injured in an accident on Highway 404 near Toronto.

A bus carrying about 50 people had an accident east of Toronto on Highway 401 near Bowmanville. Some of the passengers were injured and taken to hospital.

On Wednesday, a man died after being struck by a garbage truck while snowblowing his driveway in Sudbury, Ont.

Freezing rain pelted Quebec and parts of eastern Ontario and was expected to continue falling on Thursday from Quebec City to Ottawa.

Rail passengers were facing some delays also, according to Via Rail Canada.

Extreme cold hits Prairies, North

Elsewhere, the prairie provinces endured extreme temperatures. A cold front hit Saskatchewan and Manitoba, where temperatures dipped to –40 C.

Winnipeggers faced a second wind-chill warning in as many days. Temperatures fell to –42 C overnight and were expected to hover near –27 C during the day.

By contrast parts of the Yukon were experiencing unseasonally high temperatures, among the warmest in the country. The mercury rose to 12.9 C overnight in one community in the southern part of the territory.

In Whitehorse, the temperature is forecast to drop from 7 C on Thursday to a high of -20 C on Christmas Day.

The storm caused havoc in the United States as it moved north from the Gulf of Mexico, leaving at least 11 people dead in its wake before arriving in Canada.