A heavy snowstorm is blowing through Toronto and is expected to dump up to 25 centimetres of the white stuff on the area by Wednesday morning.

Environment Canada issued a winter storm watch Monday, which was upgraded Monday evening to a warning.

In a statement, the weather agency provided more details on the approaching storm, which hit southern Ontario Tuesday evening.

Snow began to fall in areas affected by the warning starting late this morning, and intensified in the evening. The storm is expected to be heaviest between 6 p.m. Tuesday and 2 a.m. Wednesday.

Environment Canada forecasts between 15 and 25 centimetres of snow to fall on the areas under the warning by Wednesday.

It warned that strong winds will also blow through exposed areas, with gusts as strong as 70 km/h in what could be the most severe winter storm to hit southern Ontario this year. The weather agency predicted a high of -3 C in Toronto, but with the wind chill it will feel more like -10. Overnight, the wind chill will feel like -15.

"Consider postponing non-essential travel until conditions improve," the statement advised.

"Visibility will be suddenly reduced to near zero at times in heavy snow and blowing snow. There may be a significant impact on rush hour traffic in urban areas."

During the storm, Toronto's two 24-hour cold weather drop-in centres will be open, and TTC tokens will be available at some centres.

In response to the weather, several after-school activities have been cancelled:

  • Toronto Catholic District School Board - night school
  • Durham District School Board - community use of school permits, continuing education night classes, learn-at-home guidance appointments
  • York Region District School Board - continuing education classes and community use of school permit activities
  • Peel District School Board - continuing education courses, evening events, permits, night school, ESL adult programs

Childcare programs remain open for families in the PDSB.

Centennial, Humber, and Sheridan colleges and McMaster University also closed early on Tuesday because of the forecast.

Humber College may remain closed on Wednesday depending on the severity of the storm.

City prepared for heavy snow

City of Toronto officials said snowplows would be deployed on city streets starting at approximately 9 p.m. on Tuesday, when the snowfall was expected to intensify.

Snowplows will also star clearing expressways once there is at least two centimetres of snow on the road. Crews also applied brine and salt throughout the city on Monday.

“The city has about 25 city staffers that are patrolling the roads 24/7,” said road operations manager Hector Moreno. “They’re equipped with high-end equipment that actually helps them gauge what sort of temperatures we’re sitting at.”

Transportation officials are also tracking the storm from their operations centre.

The city hopes to have snow cleared of the main roads by Wednesday morning, but residents will likely wake up to accumulated snow on residential streets.

Residents are also asked to shovel the snow off the sidewalks in from of their homes to make it safer for pedestrians.

Flights, regional transit could be delayed

The storm has already caused dozens of flight delays and cancellations at Toronto Pearson Airport.

Further disruptions are expected as the weather worsens overnight.

Airlines are asking passengers to check their flight status online or by phone before arriving at the airport.

Metrolinx is also warning commuters who rely on GO Transit to look up their train or bus schedule on Wednesday morning in cause the storm causes delays or other disruptions.

The following areas fall under Environment Canada's winter storm warning:

  • Toronto
  • Burlington-Oakville
  • Halton Hills-Milton
  • Mississauga-Brampton
  • Newmarket-Georgina-Northern York region
  • Vaughan-Richmond Hill-Markham
  • Uxbridge-Beaverton-Northern Durham region
  • Dufferin-Innisfil
  • Barrie-Collingwood-Hillsdale
  • Caledon
  • Hamilton-Niagara
  • Oxford-Brant
  • Bancroft-Bon Echo Park
  • Orillia-Lagoon City-Washago
  • Peterborough-Kawartha Lakes
  • Kaladar-Bannockburn-Bon Echo Park
  • Plevna-Sharbot Lake-Western Lanark County
  • Cornwall-Morrisburg
  • Ottawa-Gatineau
  • Prescott and Russell
  • Smiths Falls-Perth-Eastern Lanark County
  • Sarnia-Lambton
  • London-Middlesex
  • Huron-Perth
  • Waterloo-Wellington
  • Saugeen Shoers-Kincardine-Southern Bruce County
  • Hanover-Dundalk-Southern Grey County
  • Owen Sound-Blue Mountains-Northern Grey County
  • Elgin
  • Simcoe-Delhi-Norfolk
  • Dunnville-Caledonia-Haldimand

The following areas fall under a snowfall warning, with an estimated accumulation of 15 to 20 centimetres:

  • Pickering-Oshawa-Southern Durham region
  • Belleville-Quinte-Northumberland
  • Stirling-Tweed-South Frontenac
  • Brockville-Prescott
  • Westport-Charleston Lake
  • Merrickville-Wolford-Kemptville
  • Midland-Coldwater-Orr Lake
  • Bruce Peninsula-Sauble Beach-Tobermory
  • Bancroft-Hastings-Highlands-Denbigh
  • Port Carling-Port Severn
  • Bracebridge-Gravenhurst
  • Haliburton-Minden-Southern Haliburton County
  • Renfrew-Pembroke-Barry's Bay

With reports from CTV Toronto’s Colin D’Mello, Ben Mercer and Austin Delaney