A heat warning is in effect for the Barrie, Collingwood and Hillsdale areas.
According to Environment Canada, a hot and humid airmass is forecast to move in Monday and is expected to last until Wednesday.
The weather agency says daytime temperatures are expected to reach 31 C, with the humidex near 40 C. Overnight there will be little relief, with temperatures expected to drop only to the low 20s.
According to Environment Canada, relief will come on Wednesday afternoon, when a cold front is expected to move through.
On Wednesday the daytime high is expected to reach 25 C with an evening low of 15 C.
Environment Canada issues heat warnings when the weather will pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.
The weather agency warns that young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors are more susceptible to extreme heat and should exercise caution when outside.
Residents are reminded to check on older friends, family and neighbours, and to never leave pets or people inside parked vehicles.
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