The warning is in effect for Toronto, Vaughan, Richmond Hill, Markham, Mississauga, Brampton, Oakville, Burlington, Halton Hills, Milton, and Hamilton.
“Hot and humid conditions are expected Friday and Saturday,” the weather agency said.
“Daytime high temperatures reaching the low thirties with humidex values in the upper thirties are expected Friday and Saturday.”
Environment Canada said it’s possible that the “heat event” could extend into Sunday.
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Overnight lows near 20 C will “provide little relief from the heat,” the weather agency said.
It warned that the conditions could also bring deteriorating air quality.
Residents should take precautions due to an elevated risk of heat-related illness.
The Toronto area was also covered by a heat warning earlier this week.