"Two years ago we got 10 centimetres and we may get that much snow in the GTA (Wednesday)," Environment Canada meteorologist Arnold Ashton said.
"It's pretty nasty anywhere you go. Basically the whole [Highway] 401 corridor" will be affected, he said, though conditions are expected to return to normal by Thursday.
Environment Canada is forecasting that the storm, which is part of a system moving north from Kentucky, will continue towards Quebec.
The GTA will be spared from the heaviest of the snow, CBC meteorologist Claire Martin noted.
But Environment Canada has issued winter storm warnings for the Niagara peninsula, including Niagara Falls, Welland and St. Catharines.
"It's once we get into Thursday and the snow is on the ground and people start to move around, that's when we'll really start to see the effects of this storm," Martin said.
"Around the [Golden] Horseshoe, Hamilton, St. Catharines, Niagara, you’re going to pick up about 10 centimetres of snow and that's where the roads will be the worst," she said.
Parts of southwestern Ontario and the stretch between Kingston and Cornwall in the east will get the brunt of the storm. Residents there will likely be shovelling 15 to 20 centimetres of snow from their driveways Thursday morning.
Wind will be a factor for drivers, with gusts in some areas reaching up to 70 km/h.
"We'll see some snow and blowing snow on those open-area highways," Martin said.
As of Wednesday evening, more than 100 incoming and outgoing flights were listed as cancelled at Pearson International Airport ahead of the storm system approaching southern and eastern Ontario.
Travellers are advised to check out the Toronto Pearson International Airport website for the most up-to-date flight listings information.
The City of Toronto has been tracking the storm since Christmas Eve and salt truck driver Calvin Foote said he's ready to plow through whatever Mother Nature brings.
"Everything's ready. We got the plows hooked up, the salt trucks are all fuelled up, ready to go," said Foote, who is among more than 1,000 workers on standby to help clear the roads.
Hills and bridges have already been treated with liquid salt, and snowplow drivers have orders to deploy as soon as two centimetres of snow have accumulated on expressways.
In the meantime, the Ontario Safety League is warning commuters to be smart if they feel they absolutely must drive in the storm. That means going through a winter driving checklist and ensuring auto fluids like antifreeze are topped up before you travel, brushing off headlights and car rooftops, and making sure all the necessary windows are cleared of snow before you leave your driveway.
"The speed limit sign is for ideal conditions. Tonight (Wednesday) is not going to be ideal conditions," said OSL spokesman Brian Patterson.
Patterson said most drivers aren't prepared for the first blast of winter weather. The most effective way to stay safe while driving is to drive at a moderate speed and keep a keen eye out for pedestrians.
Thursday morning commuters are also being reminded not to try to pass snowplows as they clear the streets.