The City of Toronto has issued an extreme cold weather alert, with temperatures expected to fall sharply overnight and stay cold throughout Tuesday.

The forecast calls for a high of only –11 C on Tuesday, a drop that follows a run of warmer-than-seasonal temperatures through the Christmas holidays.

"With the wind chill, it's going to feel even, at the height of the day, about –25 or so, and that's about as good as it gets," CBC meteorologist Nick Czernkovich said.

When an alert is declared, homeless people are advised to seek shelter. Outreach workers will patrol the streets, shelters will reduce restrictions on service and transit tokens will be provided so people can travel to shelters.

"It is going to come as a bit of a shock," Czernkovich said. "If you think back to December, our temperatures in Toronto were about four degrees above average."

Czernkovich reminded people to dress properly to prevent frostbite but said the weather will warm up soon.

"The whole system's not lasting very long," he said. "The Arctic air is going to pull out by the time we hit Wednesday."

After a relatively warm week, some areas of southern Ontario were warned Monday to expect severe weather, with up to 40 centimetres of snow possible in the London, Woodstock and Stratford areas.

Snow squall warnings

Environment Canada also issued snow squall warnings for areas near Lake Huron and Georgian Bay, with snow expected to fall throughout the night.

"Snowfall amounts of 25 cm are possible over the Collingwood and Innisfil areas," the weather agency said. "For the Huron County, London and eastern Lambton County areas, total snow amounts of up to 40 cm are possible if one or more of the snow squalls off of southern Lake Huron become stationary for more than six hours."

Drivers were warned to leave extra time for trips and to expect reduced visibility.

Snow squalls are created when cold, Arctic air sweeps over unseasonably warm Great Lake waters.

"They're pretty localized phenomena. … It really intensifies as you get closer to Lake Huron and Georgian Bay," Czernkovich said.