From Environment Canada:

A period of very cold wind chills is expected.

Hazard: Wind chill values near minus 40.

Timing: Tonight into Friday morning. Extreme cold puts everyone at risk. Risks are greater for young children, older adults, people with chronic illnesses, people working or exercising outdoors, and those without proper shelter.

Watch for cold related symptoms: shortness of breath, chest pain, muscle pain and weakness, numbness and colour change in fingers and toes. Cover up. Frostbite can develop within minutes on exposed skin, especially with wind chill. If it’s too cold for you to stay outside, it’s too cold for your pet to stay outside.

Extreme cold warnings are issued when very cold temperatures or wind chill creates an elevated risk to health such as frost bite and hypothermia.