Environment Canada has issued a Heat Warning for a number of communities including Bracebridge and Gravenhurst.
There is a hot and humid airmass that has moved into Southern and Central Ontario on Sunday. It is expected to stay in place through Monday.
Temperatures will hit a predicted maximum of 31 C and with humidex values factored in that temperature rises to near 40. The thermometer is expected to fall to just 20 C overnight.
The warning will likely end on Tuesday when a cold front is expected to move through the area.
In a heat warning the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.