Environment Canada says that with the wind chill, it will feel like -25 C in Toronto overnight.
On Tuesday, the forecast indicates the expected high temperature in the morning will be -10 C, with the wind chill making it feel like -27 C.
The extreme cold weather alert, which helps trigger extra services for vulnerable people living on the streets, was issued Monday and is in effect until further notice.
Members of the public are asked to call 311 if they see vulnerable people out on the street who appear in need of assistance.
CBC meteorologist Jay Scotland reports that "bitterly cold temperatures" will affect much of the Greater Toronto Area as well.
Click on the video at the top of this page to see what is in store around the GTA over the next few hours.