Environment Canada has the GTA and much of southern, central and Eastern Ontario under a winter storm warning, the first of the season for Toronto.
The snow was to begin falling Tuesday night in Toronto and reach Ottawa by dawn on Wednesday, it said.
"Strong easterly winds near 60 km/h will accompany the snow tonight and Wednesday significantly lowering visibilities in blowing snow. Periods of heavy snow are also likely," it said.
Snow accumulations of 10 to 15 centimetres are possible, with 20 cm and more towards the Ottawa Valley.
On Wednesday in Toronto, the snow will change to a rain/snow mix to rain, as daytime highs of 5 degrees Celsius are expected.
CTV Toronto weather anchor Tom Brown described the storm as huge and complex, extending from the plains state of South Dakota down to Alabama on the Gulf Coast. It has also grabbed moisture from the Pacific coast.
"It's going to be a crazy 36 hours," he said.
The city of Toronto has a mountain of salt piled up ready for the roads. Among the equipment on standby:
- 600 snowplows
- 300 sidewalk plows
- 200 salt trucks
The salt trucks will go out first.
Private contractors will be taking care of some institutions such as hospitals. Ted Tyrell and his crew will be handling seven health-care facilities.
The priority is to make sure there's no ice or snow blocking wheelchair access, he told CTV Toronto on Tuesday.
"As soon as the weather hits ... instantly everyone is notified by email and a pager, and they know to report in to their site," he said, adding they will work through the night.
With a report from CTV Toronto's Alicia Markson
