A heat warning is still in effect for London, Windsor-Essex, and Chatham-Kent.
The weather agency says parts of the region could feel like 40 degrees with the humidex.
City officials also issued a heat warning for Toronto on Sunday.
Overnight temperatures are only expected to fall between 20 C and 24 C, providing minimal relief from the heat.
High temperatures mean big health risk for young children, pregnant women, the elderly, people with chronic diseases and those who work outdoors.
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There are 11 cooling centres located across the city and hours vary.
Environment Canada issues heat warnings when very high temperature or humidity conditions are expected to pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.
Last month, Edmonton Emergency Services launched an awareness campaign to remind people "a vehicle is not a babysitter".
Halifax's heat warning was expected to linger until Wednesday or Thursday, with daytime highs around 30 C and maximum humidex values as high as 40 expected until then.