TORONTO -- 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.
