Environment Canada's weather advisory also warned of the damage that can occur during severe conditions.
Environment Canada has issued a severe thunderstorm watch for London-Middlesex, Chatham-Kent, Leamington, and Windsor-Essex.
Environment Canada has issued a heat warning for Toronto and most of southern Ontario. Gusting winds can send objects airborne, damage buildings, knock down power lines or snap tree branches.More news: Former Sinn Fein leader Gerry Adams has 'explosive' thrown at his home
Some of these thunderstorms may be severe with torrential rain, damaging wind gusts, and large hail possible.
A heat warning is also in effect for the Sunshine Coast, Howe Sound and Metro Vancouver.
In Port Alberni, where high temperatures are expected to reach the mid-30s, the Port Alberni Shelter Society is providing water and places to cool down, and executive director Wes Heweitt is asking people to watch out for each other.More news: Official photo released from christening of Prince Louis
Those who may not have air conditioning are encouraged to go to a shopping mal or public library, as "even a few hours spent in air conditioning can help your body stay cooler when you go back into the heat".
Environment Canada is expecting the storms will diminish and pass through by this evening, but not before ushering in some potentially unsafe weather.
The agency is also reminding those in the area not to leave any person or pet inside a vehicle for any period of time. With the humidity, the temperature Monday will feel more like 40 C.More news: Michelle Obama spotted at Beyonce and Jay Z's concert in Paris
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