Toronto's chief medical officer has issued an extreme cold weather alert for the city as winter settles in for the long haul. 

Forecasters predict that the temperature will plunge to -10 C overnight, itself not cold enough to trigger a warning. However, it will feel closer to -20, one of the key criterion for an alert. 

"Those most at risk of cold-related illness are people experiencing homelessness or those under-housed, those who work outdoors, people with a pre-existing heart condition or respiratory illness, elderly people, infants and young children," a statement from the city said.

"People with heart problems can experience worsening of their condition up to several days after cold weather occurs."

Shelters throughout Toronto will loosen restrictions in response to an extreme cold alert, and 'outreach' teams spend the night trying to get vulnerable people into a safe place.

Public health officials also warn residents to:

  • Check the weather before heading outside.
  • Dress appropriately for the cold and try to stay dry.
  • Drink warm fluids (alcohol not included).
  • If you're walking outside, take brief respite inside warm buildings when possible.
  • Check on vulnerable family, friends and neighbours.

The extreme cold warning remains in effect until further notice from the city. Temperatures will feel like below -10 until Friday, when the mercury rises ever so slightly to more seasonable norms.