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A severe weather meteorologist with the federal agency said the wave extended across the provincial border with Quebec to encompass regions including Montreal, pushing daytime highs to 35 C in southern areas and 33 C in the north, with the humidex at or near 40 C.

Weiqing Zhang warned that Saturday was just the beginning of uncomfortably warm weather.

"The hottest day will be tomorrow — Sunday — and then Monday," Zhang said.

Some relief may come Monday for northern areas, Zhang added, but southern regions stretching from the Ottawa area all the way to Windsor could see high temperatures lasting into Tuesday.

Parts of Newfoundland and Labrador also received heat warnings Saturday, with near 30-degree temperatures expected in areas including St. John's and Gander. Zhang said the criteria for heat warnings in the Atlantic province is slightly lower than those for Ontario and Quebec.

Zhang said the Greater Toronto Area has had a particularly hot year, noting the current heat warning marked the region's sixth for 2025. That's compared to four warnings issued in 2024, a benchmark she said the GTA reached this past July.

She expected another heat warning could be issued later in August.

"I can't guarantee it but based on the current guidance it looks like at least in late August we'll probably have another one — the week of Aug. 20," said Zhang.

"This year the whole, the big weather pattern is favourable for hot and humid conditions."

The current heat bout will offer slightly cooler nights but it will still be warm, she added, with overnight temperatures expected to linger in the 20s. 

Zhang advised people to limit direct exposure to the sun and heat, if possible, with any outdoor activities scheduled during the coolest parts of the day. 

The weather agency warned the heat wave could pose health risks and says to watch for signs of heat exhaustion, including headache, nausea and dizziness.

°µÍø½ûÇø Canada also issued warnings of possible severe thunderstorms in the Ontario areas of Nipigon, Thunder Bay and Black Sturgeon Lake.

This report by °µÍø½ûÇø was first published Aug. 9, 2025.