Heat warnings were issued on Friday for parts of eastern Canada, southern Ontario and Toronto as hot and humid weather settles over the region and the stifling conditions are expected to last until Thursday.

Toronto is expected to reach 90 degrees on Saturday, but with high humidity levels, it could feel like it’s 104 degrees, or 40 degrees Celsius, outside, according to Environment Canada, the federal agency that provides forecasting for Canada.

Overnight lows are expected to be in the 70s.

“There’s no relief from the temperatures through the night,” said Brent Linington, a meteorologist for Environment Canada

The warning for southern Ontario was issued on Friday and covers an area between the north shore of Lakes Ontario and Erie and the Ontario-Quebec border.

The heat is expected to peak on Saturday, with temperatures dropping slightly on Sunday as a cold front passes over the area.

“It’ll be a battle between a weak cold front and the daytime heating from the sun,” Mr. Linington said of the weather on Sunday. “That’s looking like the coolest day in the stretch.”

Temperatures across southern Ontario are expected to range from the low to mid 80s on Sunday.

Temperatures are predicted to gradually increase on Monday through Wednesday, nearing 90 degrees, before a cool air mass potentially brings relief on Thursday.

This is the third major heat wave for the region since the start of the summer.

In the two previous events, temperatures were higher than those that are expected in coming days, but the previous events were shorter in duration.

This upcoming heat wave is notable because it’s expected to last longer than three days.

“With this one, the temperatures aren’t too scary but the duration is more the issue,” Mr. Linington said. “The more days that get added on to a heat event, the more hazardous it becomes.”

The heat is also expected to seep into southwest Quebec. Montreal is under a heat warning on Saturday and Sunday.

Heat warnings were also issued in Newfoundland and Labrador, where hot weather is in the forecast through Tuesday.

Amy Graff is a Times reporter covering weather, wildfires and earthquakes.

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