Environment Canada says high humidity will persist in Toronto, the GTA and Hamilton for a third straight day, with humidex values approaching 40 C.
The meteorological agency said that although temperatures are receding slightly from highs over the weekend, persistent humidity will make things feel muggy and hot until at least Tuesday.
The actual forecasted temperature in Toronto is expected to reach 28 C this afternoon, with thunderstorms possible throughout the day.
Cooler weather is expected to arrive in the region sometime on Tuesday.
The heat means everyone should drink plenty of water even when they do not feel thirsty.
Those working outdoors should take frequent breaks and the elderly and small children should limit activity outdoors.