A special weather statement from Environment Canada says a mass of frigid Arctic air will keep the mercury well below freezing into the coming work week.
Temperatures will be "well below average" for this time of year, according to forecasters, going as low as -20 C in some regions. It will be slightly warmer around the Great Lakes and in southwestern Ontario, perhaps dropping to only -15 C.
The temperature in Toronto is expected to fall to -10 C tonight, though with the wind chill it will feel more like -20 C.
A mix of winter storm, freezing rain and wind warnings remain in effect across southern Ontario following the wintry precipitation that fell across the province on Saturday.
Pearson International Airport in Toronto is reporting some delays and cancellations. Travellers are advised to check with their airlines. Flights of out Billy Bishop Airport are running mostly on time, as are GO Transit and the TTC.
Environment Canada says brisk winds and low temperatures will likely lead to extreme cold warnings during the week.
The deep freeze is part of a wave of wet, cold weather that has blanketed much of the country this weekend.