A major snowstorm pummelling southern Ontario and parts of the U.S. is forcing school and other closures as well as flight delays and cancellations, and has caused traffic woes that include at least 150 automobile accidents in the province alone.

Between 15 and 25 centimetres of snow are expected to fall across southern Ontario, with the possibility of winds of up to 60 km/h.

It's expected the storm will deliver the Greater Toronto Area's most significant snowfall in five years. CBC meteorologist Claire Martin said the last time Toronto experienced similar snow accumulations was Feb. 6, 2008, when 30.4 cm hit the city.

The same weather system is also poised to dump up to 90 cm 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 heaviest snowfall is expected to hit during the morning rush hour along the Highway 401 corridor from London, Ont., across the Waterloo region and into the Greater Toronto Area.

In Toronto, city crews have dispatched about 600 plows.

Ontario Provincial Police Sgt. Dave Woodford said that by 8:30 a.m. ET, there had been 150 collisions in southern Ontario, including the GTA, since midnight.

Traffic is light on the region's highways and only moving at about 20 km/h, he said.

"Conditions are treacherous right now," Woodford said. "People think because they have snow tires or four-wheel drive, nothing can happen to them. If you have to be out, slow down. If you don't have to be out anywhere, don't go on the roads."

The Canadian Automobile Association said it had received about 900 calls for service by early Friday morning.

Parts of southern Quebec can also expect to see snow as the system moves east toward the Maritimes. Nova Scotia, P.E.I. and southern New Brunswick are all under blizzard or blowing snow warnings.

CBC meteorologist Jay Scotland said as much as 40 cm of snow is possible in Nova Scotia.

Thousands of flights cancelled

Thousands of international flights have been cancelled in anticipation of the storm.

A city snowplow clears King Street West amidst heavy snow. Police said over 150 accidents have been reported in Southern Ontario since last night. A city snowplow clears King Street West amidst heavy snow. Police said over 150 accidents have been reported in Southern Ontario since last night. (John Rieti/CBC )

About 400 flights have been cancelled at Toronto's Pearson International Airport. WestJet pre-emptively cancelled 40 flights Thursday and Friday to avoid a backlog in its schedule as the storm intensifies.

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 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 also 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 are closed today, including the Waterloo Catholic School Board, the Waterloo Public School Board and the Peel District School Board.

The Toronto District School Board, the Toronto District Catholic School Board, as well as the York Region and York Catholic boards will remain open, but have cancelled school bus service.

York University in Toronto and McMaster University in Hamilton are also among the schools closed today.

In Ottawa and much of eastern Ontario, school buses are cancelled.

The nation's capital is expecting about 15 cm.

With files from The Canadian Press