Environment Canada says daytime high temperatures reaching the low thirties with humidex values in the upper thirties are expected Friday and Saturday.
There is a possibility that the heat event could stretch into Sunday.
Overnight low temperatures near 20 degrees Celsius will provide little relief from the heat.
The agency warns that hot and humid air can also bring deteriorating air quality and can result in the Air Quality Health Index approaching the high risk category.
Watch for the effects of heat illness: swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.
Check on older family, friends and neighbours. Make sure they are cool and drinking water
Seek a cool place such as a tree-shaded area, swimming pool, shower or bath, or air-conditioned spot like a public building.
Never leave people or pets inside a parked vehicle.
Outdoor workers should take regularly scheduled breaks in a cool place.