In a post on its website, the weather office said weekend daytime temperatures could reach 31 C with the humidex climbing as high as 42.
There isn’t much relief at night with minimum temperatures only dropping to 20 C.
The heat warning covers much of southern Ontario, spanning from Windsor to Durham Region and up to the Bruce Peninsula.
Heat warnings have been in place for much of the month so far with daily highs since Aug. 17 being in the upper 20s and lower 30s.
There is also a chance it could be extended into next week with highs expected in the 25-30 C range for much of the area.
Environment Canada adds that the high temperatures could also cause the air quality to deteriorate.
It said the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.