Environment Canada warned residents of southern Ontario and southern Quebec to brace for a possible flash freeze overnight that could lead to hazardous conditions on roads and sidewalks and delay some flights Wednesday.

The weather agency said a low pressure system has spread a mix of rain and wet snow across southern Ontario with more snow expected overnight as the temperature drops.

Late Tuesday, police said Highway 11 between North Bay, Ont., and Cobalt had been shut down because of extremely dangerous driving conditions.

The North Bay detachment of the OPP said several-tractor trailers were stuck in snow and blocking parts of the highway. The OPP couldn't say how long the stretch of Highway 11 would be shut down. North Bay is about 150 kilometres south of Cobalt.

Sault Ste. Marie, Sudbury and North Bay were all forecast to see temperatures plunge by 20 degrees. The sudden drop wasn't expected to be as severe in Windsor, London, Hamilton and Toronto, but Environment Canada cautioned that Tuesday's rain and wet snow in all those areas would probably cause flash-freezing on roads and sidewalks, creating "hazardous travelling conditions."

"The combination of the snowfall and plummeting temperatures will make for a difficult Wednesday morning commute," Environment Canada warned.

Up to 15 centimetres of snow is expected in the Ottawa area, while in Quebec as much as 40 centimetres is possible in some regions.

Air Canada and West Jet are warning customers to expect flights to be delayed due to snow in Ottawa, Toronto, Montreal and Quebec City.

Porter Airlines issued similar warnings and added Windsor, Ont., to the list of cities with potential flight delays.

There were a number of delays and some cancellations at airports in Toronto, Ottawa and Montreal on Tuesday.

The Canadian Automobile Association is warning drivers to take extra care — and has produced a list of winter driving tips.

Most of southern Ontario and Quebec have seen unseasonably balmy temperatures so far this season and little to no snow. In its long-term weather outlook issued earlier this month, Environment Canada forecast a warmer-than-normal winter for both areas.

The storm is forecast to push through all the way to the East Coast, with 40 mm of rain expected in parts of Nova Scotia Wednesday, and 35 cm of snow expected in parts of Labrador by early Thursday.

Winds of up to 130 km/h are also expected in Newfoundland's Wreckhouse area and in the Cape Breton Highlands on Wednesday.

With files from The Canadian Press