Environment Canada has issued a heat warning for parts of southern Ontario starting Thursday, with temperatures expected to reach up to 35 C.
The weather agency says a stretch of the province bordering Lake Ontario, from St. Catharines, Ont., to Toronto, will see peak humidex values of up to 44 C.
It says while daytime high temperatures will not be as extreme on Friday and into the weekend, nighttime lows are expected to remain warm, providing little relief from heat.
Environment Canada says temperatures on Friday will near 30 C, with a humidex of up to 40 C.
It's not the first heat warning for Ontario this summer — extreme heat reaching the mid-30s broke temperature records in many parts of the province in June.
During extreme heat events, people are advised to drink water often, watch for signs of heat exhaustion and to check on older adults and those at risk of heat illness.
This report by The Canadian Press was first published July 23, 2025.
The Canadian Press
