Much of the GTA is bracing for a significant temperature plunge overnight and early Friday with extreme cold weather alerts issued by Environment Canada.

The alerts are prompted by forecast temperatures of –21 C or colder, or an expected wind chill of –30.

"Bitterly cold wind chills near –30 are expected overnight and into Friday morning," the federal weather agency said.

"Keep emergency supplies in your vehicle such as extra blankets and jumper cables. Outdoor workers should take regularly scheduled breaks to warm up."

Extreme cold warnings are issued when very cold temperatures or wind chills create an elevated risk to health, such as frostbite and hypothermia.

Toronto's Medical Officer of Health Dr. Eileen de Villa says residents should help vulnerable people by calling 311 if there is a need for street outreach assistance, and call 911 if the situation is an emergency.

Winter services for homeless, under-housed people

During the alert, Metro Hall on John and Wellington streets will host a warming centre, continuing until noon on the day the alert is ended.

Other services activated during the alert include expanded services at community agencies, transit tokens and tickets for vulnerable people, and more street outreach.

Meanwhile, York Region's Medical Officer of Health Dr. Karim Kurji said people should limit outdoor activities. Those who must be outside should dress appropriately in layers, wear hats, neck warmers, gloves, socks and windproof and waterproof clothing, she said,

"Hypothermia is a serious health condition that happens when body temperature drops dangerously low, starving your brain of oxygen and affecting your ability to think clearly or move well," Kurji said.

"Don't ignore shivering and look for the 'UMBLES' (mumbles, stumbles or fumbles), which may indicate hypothermia."

The City of Markham has also issued an extreme cold warning beginning overnight Thursday, as wind chill values are expected to fall to near –30 in the city.

The extreme cold warning is expected to remain in effect into Friday morning, the city said in a statement.