Accidents that have caused injuries have already been reported.
Across most of southern and eastern Ontario, the system has dumped the season's first load of heavy, wet snow, with more expected through western Quebec.
As of 8 a.m. ET, 24 centimetres were recorded in the Ottawa area, much of the snow falling overnight. It should taper off by midday.
"In Montreal, 12 centimetres of snow was reported at Trudeau International Airport, but it'll switching over to rain for a combination of rain and wet snow," said CBC meteorologist Jay Scotland.
"There is the risk of freezing rain through the Eastern Townships," he said.
"Areas north and east of the St. Lawrence are seeing this as a snow event, with 15 to 25, upwards of 30 centimetres of snow possible, including what fell last night."
Snow and slush are making for dangerous driving conditions. In western Quebec, an armoured Brinks truck overturned early Wednesday on a Gatineau expressway, sending one person to hospital and injuring two others. Elsewhere, a car slid into the Gatineau River, sending a woman to hospital with hypothermia.
All buses assigned by Eastern Ontario schools boards have been cancelled.
Farther to the east, Environment Canada says wind gusts of up to 100 km/h are in the forecast Wednesday for parts of Nova Scotia, New Brunswick, Prince Edward Island and western Newfoundland.
In New Brunswick, a mixture of light snow and freezing rain in the early morning hours closed schools in the Fredericton and Oromocto areas.
U.S. storm slows holiday travel
The same storm system moving across Eastern Canada is also south of the border, a day before the American Thanksgiving Day holiday. About 40 million people are on the move for one of the busiest travel days in the U.S. Most of them are on the road, but an estimated three million move through airports to visit relatives.
The CBC's Meagan Fitzpatrick, reporting from Reagan International Airport in Washington, said the runways there are "really slick."
"It has been raining non-stop since 9 p.m. Tuesday and the rain is to continue all day on Wednesday.
"High winds are expected and could also present problems in the northeast part of the country," said said, adding that cancellations and delays at Reagan International are affecting arrivals more than departures.
The affected flights include an Air Canada flight from Ottawa that has been cancelled.