Environment Canada issued weather statements for parts of southern Ontario on Monday, noting that it was expecting the “first heat event of the season.”
The agency says the high temperatures can pose a health risk to people not used to the heat.
The weather statements affect a region that stretches from Windsor to east of Toronto.
Environment Canada says a warm and humid air mass is expected to move into the region, bringing temperatures higher than 30C, with a humidex of nearly 40.
Overnight, temperatures are expected to drop to about 20 C.
Environment Canada is advising people to drink plenty of cool liquids before feeling thirsty and spend a few hours each day in a cool place.