A heat warning has been issued for London and Middlesex County.
Beginning Sunday, Environment Canada says the humidex value will reach 40 degrees.
Counties surrounding Middlesex, including Lambton, Elgin, Oxford and Huron are also under an alert.
A warm front is expected to bring a southwesterly flow of hot and humid air over southern Ontario.
Current information suggests daytime highs near 31 degrees and low temperatures near 21 degrees are likely on Sunday and Monday.
Humidex values near 40 are possible with little night time relief from the heat.
A cold front is expected to move over southern Ontario on Monday night to end the heat event.
The agency says extreme heat affects everyone.
The risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.
It cautions to never leave people or pets inside a parked vehicle.
