Dozens of flights through Toronto Pearson and Montreal-Trudeau airports have been delayed or cancelled.
Ontario Provincial Police are cautioning motorists to take care on highways, posting on Twitter — "IF YOU SEE SNOW, GO SLOW!"
The OPP says weather may have been a factor in a head-on collision that killed two people near Barrie, north of Toronto.
Police also say there have been numerous accidents including a 20-car pileup on the Queen Elizabeth Highway in Mississauga.
Environment Canada is forecasting a significant snowfall for parts of southern Ontario by early Sunday morning, with an estimated 15 to 25 centimetres in some areas.
The storm system will move through the Maritimes on Sunday, and winter storm warnings are in effect.
Snowfall amounts of between 30 and 40 centimetres are forecast for southern New Brunswick, with slightly lesser amounts predicted for Prince Edward Island and Nova Scotia.
Environment Canada forecaster Rosemary Tabory says the weather has caused the biggest problems in southwest Ontario.
”So far the worst-hit areas have been Windsor because of snow coming off of Lake Erie to the Toronto and Hamilton areas as well along the lakeshore ... again because of snow coming off Lake Ontario,” she said.
The storm was expected to pick up steam as it moved eastward overnight and into the Maritimes on Sunday.