Most of the province is under an extreme cold warning.
Environment Canada says parts of northern Ontario will see wind chills as low as minus 45 today.
In the south, forecasters say some areas will see wind chill values of minus 40 improve by this afternoon, but fall below minus 35 again tonight.
Extreme cold warnings are issued when the risk of frostbite and hypothermia are high.
Provincial police are reminding people travelling during the Family Day long weekend to drive according to the changing road conditions.
They say snow squalls and extreme cold conditions could make for some difficult driving conditions.
Snowmobilers are also urged to check with local ice fishing operators and snowmobile clubs before venturing out onto frozen waterways.
