The city's medical officer of health extended an existing alert.
As of noon on Sunday, the temperature hovered around –6 C, with a wind chill that was making it feel more like –11.
But Environment Canada says the temperature will fall overnight.
The weather agency says it is possible that a combination of cold arctic air and wind could make it feel between –30 and –40 overnight.
These same conditions will persist into Monday morning. Tomorrow afternoon, things will improve as "temperatures moderate," Environment Canada says.
Toronto is also seeing some light snowfall on Sunday.
City crews are also out clearing streets following the snowfall that hit Toronto on Saturday.