The city's acting medical officer of health has issued an extreme cold weather alert for Toronto as temperatures dip well below freezing.

The warning will remain in effect until further notice.

The city issues the alerts when the temperature is forecast to hit -15 C or colder, or when the wind chill is expected to reach -20 or colder.

<a href="https://twitter.com/hashtag/CityofTO?src=hash">#CityofTO</a> Acting MOH calls Extreme Cold Weather Alert for Toronto - in effect until further notice. Details: <a href="https://t.co/lA8HRG7MWx">https://t.co/lA8HRG7MWx</a> —@TorontoComms

But it won't quite be that cold during the day on Thursday, according to Environment Canada.

The federal agency is forecasting a high of -3 C on Thursday, with a 40 per cent chance of flurries. Wind gusts could hit 50 km/h through the morning. The wind chill as of 8 a.m. was -11.

The overnight low, however, is expected to reach -11 C. A high of -5 C is forecast for Friday with a 40 per cent chance of flurries.

The city's alert notes that exposure to cold weather can have serious health consequences, including hypothermia and frostbite.

Such alerts activate local services that help get vulnerable residents indoors, including boosting the number of shelter beds, the relaxing of service restrictions at community agencies, overnight street outreach and free tokens at some drop-in programs.

Residents are advised to dress in layers, cover exposed skin, wear wool or synthetic layers close to skin to stay warm and check on vulnerable neighbours and friends.