Nova Scotia, Prince Edward Island and southern New Brunswick are all under blizzard or blowing snow warnings. Newfoundland is under a winter storm watch for Saturday afternoon.
The worst weather is expected to hit the Maritimes overnight and into Saturday. CBC meteorologist Jay Scotland said as much as 40 centimetres of snow is possible in Nova Scotia, and wind gusts could reach a maximum of 100 km/h.
In Ontario, the winter blast caused long delays for evening commuters with more than 350 accidents reported in the province alone, and at least four weather-related fatalities.
The OPP said a 45-kilometre eastbound stretch of Highway 401 east of Brockville remained closed Friday night after a pair of collisions.
The driver of a commuter bus was taken to hospital this afternoon after his vehicle rolled over in the eastbound lanes of Highway 401 near Brockville, 115 kilometres south of Ottawa. The OPP said some of the 38 passengers also suffered minor injuries.
OPP had reported earlier that the crash was fatal, but east region spokeswoman Sgt. Kristine Rae said later that there were no fatalities.
A 57-year-old Ottawa man died Friday morning near Prescott as a result of a collision, and a 23-year-old woman was killed on Highway 112 in Marieville, Que., after losing control of her vehicle and colliding with another vehicle.
Durham regional police said a 49-year-old Oshawa, Ont., man was killed as a result of a multi-vehicle collision in Pickering, east of Toronto, on Friday morning.
The CBC's Weather Centre said 20 to 30 centimetres of snow fell on the Greater Toronto Area by 4:30 p.m ET, making this the biggest storm the city has seen since 2008.
Other areas of Southern Ontario could see upwards of 35 centimetres of snow, with winds gusting up to 50 km/h.
Environment Canada said areas of southwestern Quebec could expect five to 10 centimetres of snow as the storm continues to move east Friday night, with up to 25 centimetres forecasted near the borders.
The agency issued a blowing snow warning for the southern parts of the province, saying wind gusts could reach up to 60 km/h.
Snow removal delayed
Transportation officials said many collisions occurred near highway off- and on-ramps, preventing snowplows from clearing the highways.
"We must restrict travel on the highways so the snow removal can be done properly," OPP said in a statement.
Paramedics in Hamilton said an 80-year-old woman died after collapsing while shovelling her driveway this morning.
The same weather system is also poised to dump up to 90 centimetres of snow across the northeastern United States.
Environment Canada is warning commuters of hazardous travelling conditions due to near-zero visibility, and heavy and blowing snow.
The snow in the Toronto area will end in the evening and overnight in easternmost sections of the province, the weather agency said.
In Toronto, city crews have dispatched about 600 plows.
The Canadian Automobile Association said it had received over 4,500 calls for service by early Friday evening.
Thousands of flights cancelled
Thousands of flights have been cancelled not only due to the storm hitting Ontario, but because of poor weather conditions from Chicago to Quebec City.
About 800 flights, or nearly half, have been cancelled at Toronto's Pearson International Airport.
Travellers at airports across Canada are being advised to call airlines ahead of time to check if flights have been cancelled or delayed.
In the U.S., New York's LaGuardia, Newark's Liberty, Boston's Logan and Chicago's O’Hare are among the most affected airports, with more than 3,700 flights cancelled in the U.S.
Ontario commuters face delays
Ontario's GO Transit is advising its customers to leave extra time for travelling and to check its website for service updates. It is warning that customers may experience delays on its bus and train systems.
Several school boards across southern Ontario closed today, including the Waterloo Catholic School Board, the Waterloo Public School Board and the Peel District School Board.
York University in Toronto and McMaster University in Hamilton were also closed today. The University of Toronto closed its Mississauga and Scarborough campuses but its main downtown campus stayed open.
Buses were also cancelled for the Toronto District School Board, the Toronto District Catholic School Board, as well as the York Region and York Catholic boards.
On the streets of Toronto, many people were trudging along sidewalks covered in snow as they made their way home from work.
"It's not easy going but we have an office we have to keep salted up," said Eddie Sobo, a construction worker who was struggling to push a wheelbarrow full of road salt through the snow.
Sobo said he's not averse to dealing with snow on the job and he needs to keep surfaces salted for office staff who "need to get out."