The massive weather system also grounded flights, totalled cars and shut down schools, foreshadowing a wild weekend for Atlantic provinces that are next in the storm's path.
The storm churned its way eastward throughout the day, painting Ontario white from Windsor through to Ottawa.
Environment Canada said most regions will find themselves under 15 to 25 centimetres of snow by day's end.
"The amounts do vary, but no one's been left out of the snow on this one," senior climatologist David Phillips said in a telephone interview. "I think it has followed through just as we thought."
The storm didn't take long to claim its first victim. Ian Wright of Hamilton Paramedic Services said an 80-year-old woman in that city collapsed while shovelling her driveway early in the morning. She was pronounced dead on the scene, he said.
The burst of snow caused numerous accidents on the province's roads, which accounted for at least two other deaths.
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.
Further east, Provincial police reported a 57-year-old Ottawa man died when his car crashed in blizzard conditions along Highway 401 near Prescott, Ont.
Also on Highway 401, a bus driver and some passengers suffered injuries the vehicle lost control and rolled near Brockville in eastern Ontario.
Ontario Provincial Police Sgt. Kristine Rae said the driver of the bus received serious injuries, while she described the injuries to the passengers as minor.
Rae couldn't say if the weather caused the crash but said it was snowing at the time that the bus carrying 38 passengers went off the road.
Provincial police Sgt. Dave Woodford said treacherous driving conditions shut down stretches of highway from Chatham to Brockville throughout the day.
Much of the traffic-related trouble was centred in Toronto, he said, where more than 350 collisions had been reported since midnight.
"We're urging people to stay off the roads at this time so we can get the highways cleaned up," he said.
The spike in accidents came as little surprise to Woodford, since Torontonians have grown accustomed to nearly bare streets during four consecutive winters without significant snowfall.
Despite contending with the largest storm since December 2008, however, most appeared to be taking the weather in stride.
Bike courier Brendan Bar was undaunted by the prospect of stashing his vehicle in snowbanks as he made his rounds, nor by the idea of wheeling himself down streets that had not yet been plowed.
"It's not that bad," he said as he wiped ice crystals from his beard. "We do it all year long so we're used to it."
For construction worker Eddie Sobo, the snowy weather added extra incentive to maintain business as usual.
"It's not easy going but we have an office we have to keep salted up," he said as he struggled to push a wheelbarrow full of road salt through the snow.
Passengers flying out of Toronto's airport faced a raft of cancellations as Ontario's wintry weather and a massive storm dumping nearly a half-metre of snow on parts of the U.S. combined to scuttle flights.
Schools and universities were also feeling the effect of the storm.
In Toronto and the regions of York, Durham and Simcoe, all public and Catholic schools remained open but buses were cancelled. Schools were closed in Peel, Halton and Hamilton-Wentworth Regions. In the Ottawa area, schools were open but buses cancelled.
Classes were cancelled at several universities and colleges, including York and Ryerson Universities in Toronto, McMaster University in Hamilton, Brock University in St. Catharines and the University of Guelph.
Social media was abuzz over the snowy conditions, though gripes about the slew of cancellations were greatly outnumbered by gleeful comments about the return of winter.
Twitter users in Toronto documented inconveniently large snow drifts, impromptu snow sculptures and even pedestrians navigating snowy sidewalks on skis.
Others took time to offer advice to their fellow snowbound citizens.
"Stay off the roads! Hot cocoa and pyjamas will have to suffice," wrote one user.
Phillips said there are rewards in store for Ontarians who weather the storm. Friday's snowy conditions are forecast to give way to sunny skies for the weekend, giving people a chance to take advantage of the new precipitation.
"We're cursing it now, but we'll bless it tomorrow when we have that sunshine," Phillips said. "The beauty of it will be evident to us through Saturday and Sunday."
Ontario's storm is expected to pale in comparison to the one bearing down on the east coast, Phillips said. Quebec is expected to emerge comparatively unscathed, but parts of New Brunswick, Nova Scotia and Newfoundland should brace for between 30 and 40 centimetres of snow and winds gusting up to 100 kilometres an hour, he said.
Atlantic Canadians can expect to start feeling the storm's effects by Saturday.
—With files from Peter Lozinski
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