A major snowstorm pummelling Ontario on its way to the Maritimes has left at least four people dead.

The storm, which began slowly overnight, intensified during Friday morning's commute, with the worst of it hitting the Highway 401 corridor between London, Ont. and the Greater Toronto Area, as well as south through Niagara and the Golden Horseshoe.

Early Friday morning, an 80-year-old woman died while shovelling snow in Hamilton. In Pickering, Ont., a 49-year-old man from nearby Oshawa lost his life in a three-vehicle collision.

Another crash, this one along Highway 401 near Prescott, Ont., killed a 57-year-old man from Ottawa. And in Marieville, Que., a 23-year-old woman died in a car crash, according to La Presse.

A bus carrying 38 people also rolled on the 401 near Brockville, Ont., seriously injuring the driver. Some passengers suffered minor injuries, police said. A stretch of the highway was closed for some time after the crash.

Ontario Provincial Police reported about 350 accidents in the Greater Toronto Area alone since midnight Friday.

By Friday evening, the Canadian Automobile Association received nearly 3,000 calls for help, as many drivers headed out onto the roads despite the forecasts and warnings. Plenty of others decided to stay home in a self-imposed “Snow Day,” spending the morning shovelling out their walks.

A number of school boards cancelled school buses while others cancelled classes altogether.

A snowfall warning remained in effect Friday evening for almost all of southern Ontario. Environment Canada predicts 15 to 25 centimetres will fall on Toronto by day’s end.

Areas east and west of the city could be digging out from as much as 30 centimetres of snow. Some freezing rain and ice pellets are possible over the extreme southwest.

The storm is also moving east into Kingston, Ottawa and Montreal. Many of those areas won't see as much snow as the southern part of the province, but are already seeing plenty of high winds.

The snowfall was not expected to slow down until the evening in the Toronto area. It will ease in easternmost parts of Ontario and southern Quebec overnight.

The storm is the result of a double-whammy of an Alberta clipper and a Texas low that collided over southern Ontario. The low pressure system is expected to head out to the Maritimes Friday night, where it will merge with another storm that's currently affecting parts of New England.

That storm will intensify as it tracks south of Nova Scotia Saturday, Environment Canada warns, bringing heavy snow, winds of up to 80 km/h, and blowing snow to southern New Brunswick, P.E.I., and many parts of Nova Scotia. The blizzard conditions in the Maritimes are forecast to begin near midnight Friday, with a total snowfall of up to 30 centimetres by Saturday evening.

Many flights in and out of Toronto were cancelled Friday morning, with Toronto's Pearson International Airport is urging passengers to check flight information before heading out.

Still, hundreds of travellers ended up waiting in long lines at the airport -- and even on the tarmac. Some people complained on social media that they were forced to sit in grounded airplanes for hours, only to find out that the flight had been cancelled.

With files from The Canadian Press and The Associated Press