People in southern Ontario are being warned to expect another wintry blast, as a major snowstorm passes through the region Thursday and Friday.

Environment Canada has issued a winter storm watch for much of southern and parts of eastern Ontario, extending from London to Peterborough, and including Toronto.

Between 15 and 25 centimetres of snow along with strong easterly winds are expected. Some freezing rain and ice pellets are also possible over the southwest.

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.

"It's been a long time since we've seen a snowfall this heavy," said another CBC meteorologist, Jay Scotland. "And we're not just looking at snow. Things will be really bad in terms of visibility for Friday morning."

Possible flight cancellations

The storm is expected to start on Thursday evening, with the bulk of the snow, about 20 cm, expected to fall between 9 p.m. and 9 a.m. ET.

The City of Toronto has issued a cold weather alert, a move that opens additional spaces at homeless shelters. City officials were also busy readying snow-clearing equipment, including plows that weren't used even once last winter.

The City of Hamilton has issued a winter storm warning, and officials say there are more than 400 snow-clearing vehicles ready to go when the storm arrives.

Pearson International Airport posted a weather alert on its website, warning that those scheduled to travel on Thursday or Friday should check the status of their flight before leaving for the airport.

"Toronto Pearson has invested in infrastructure, equipment and staff training for effective snow clearing, de-icing of aircraft and in-terminal assistance for passengers," it said in a statement.

WestJet issued a similar travel warning for Thursday and Friday, that a winter storm may cause the delay or cancellation of flights to and from Toronto.

Air Canada has issued an alert for Thursday, warning that flights may be "impacted by forecasted snow."

Difficult morning drive

That will make for a difficult Friday morning rush-hour commute anywhere along the Highway 401 corridor from London to the GTA and through the Golden Horseshoe.

City staff get ready The City of Toronto has a snow-clearing plan in place for major storms. It includes: The city has 600 snowplows, 300 sidewalk plows and 200 salt trucks.

Expressway plows are deployed when two cm of snow is on the ground, major roads at 5 cm; side streets at eight cm or whenever the snow stops falling.

About 1,500 workers will be deployed if storm materializes as predicted.

Plow drivers assigned to clear expressways and major routes are on standby. They can be deployed within three to five minutes.

Side streets and sidewalk plows are on call, deployed within two hours.

CBC News reporter Trevor Dunn said commuters should prepare for a difficult drive or arrange to take transit.

"The worst is coming overnight and it will continue through early Friday," Dunn reported Thursday. "By all appearances, tomorrow morning looks treacherous."

The storm will also bring heavy snowfall to eastern Ontario, parts of Quebec and Eastern Canada.

The storm is the combination of two weather systems that will merge in the eastern United States. Many cities in the northeast corridor are expecting blizzard conditions on Friday. Parts of New England are predicting accumulations of up to 60 cm.

Friday night's storm will be a far cry from the one that hit Toronto in January 1999, when the city famously called in the army to help with the cleanup. Thirty-eight centimetres fell in one day and more than 100 cm accumulated within a week.

Things will be quite different come Monday, when much of the weather will turn to a slushy mess.

Monday forecasts for much of Ontario are calling for rain and above-zero temperatures, including: