Environment Canada warns of high heat and humidity for southern Quebec and Ontario

Canada Day festivities are coming with a risk of heatstroke this week as Environment Canada forecasts an extended period of hot and humid weather with humidex values making it feel like more than 40 C for southern Quebec and Ontario.

The first major heat event of the summer triggered a wave of warnings on Monday, kick-starting a week of sweltering temperatures in the forecast across many of the two provinces' cities.

"The strongest part of that heat wave will be on Wednesday and Thursday, just in time for Canada Day," Environment Canada meteorologist Simon Legault said in an interview.

Legault said the heat won't be record-breaking, but it's coming earlier in the summer when people are less accustomed to it. It will also last for several days and won't cool down much at night, allowing the effects of heat to accumulate in the body, he said.

The agency has issued yellow heat warnings for Sarnia, Windsor and several other Ontario communities, predicting temperatures of up to 36 C as well as humidex values that could reach the 40s.

In Toronto, temperatures are expected to peak between Wednesday and Friday, topping out at around 35 C before the humidity is factored in.

The heat is expected to move into Montreal on Wednesday and is forecast to last until Saturday, with little relief at night. Parts of Quebec's Outaouais, Lanaudière, Laval, Laurentians, Montérégie, Centre-du-Québec and Estrie regions will also be affected.

Samantha Green, a Toronto family doctor and president of Canadian Association of Physicians for the Environment, says she's worried about some of her patients, including young children and older people who are socially isolated.

People exposed to high temperatures, she said, can develop heat exhaustion and potentially deadly heat stroke. Those with chronic health conditions, including lung disease, heart disease and mental illness, are at risk of having those illnesses worsen during extreme heat events.

"Heat is deadly," she said. "It's actually the deadliest form of extreme weather in Canada. The heat dome event that hit British Columbia in 2021 was the deadliest weather event in Canadian history and we need to protect people who are most vulnerable because these are all preventable deaths." The 2021 heat wave in B.C. was record-shattering and linked to hundreds of sudden and unexpected deaths.

As is tradition, Canada Day is also considered Quebec's unofficial moving day, when leases expire for many households and streets are lined with trucks and littered with discarded items. This year, renters will be loading and unloading moving trucks in temperatures forecast to be in the 30 to 33 C range with humidex readings up to 44.

Montreal Mayor Soraya Martinez Ferrada said the city is making sure its water parks and cooling centres are up and running, especially in places where homeless and vulnerable people gather. 

"I see what's happening in Europe," she said about the killer heat wave moving across the continent. "We've gone through this in our city. We just want to make sure that people know where to go." 

France recorded around 1,000 excess deaths last week at the height of its record-smashing heat wave, the country’s public health agency said Sunday, as several other European countries recorded their highest-ever temperatures.

Extreme heat is also one of Canada’s most dangerous climate risks. During six heat waves last summer, Toronto emergency rooms fielded 74 heat-related visits, the local health unit said. 

Montreal’s public health department says it received three reports of possible heat-related deaths during an August 2025 heat wave. That same heat wave over southern Quebec was found to be at least two to 10 times more likely due to climate change, an analysis by federal scientists suggested.

Green says heat-related hospitalizations and deaths aren't very well-tracked in Canada, suggesting those numbers could be higher.

Toronto and Hamilton are both exploring bylaws that would require landlords to cool units below 26 degrees. 

Meanwhile, new Ontario rules taking effect Wednesday will prohibit landlords from preventing tenants from installing an air conditioner, provided the tenant gives written notice and meets certain other conditions.

Environment Canada is asking people to check on older or at-risk people and to watch for early signs of heat exhaustion, which include headache, nausea, dizziness, thirst and intense fatigue. People should also seek out cool or air conditioned spaces to get relief from heat, as well as drink plenty of water, Legault said.

This report by The Canadian Press was first published June 29, 2026.

— With files from Jordan Omstead in Toronto and The Associated Press

By Morgan Lowrie | Copyright 2026, The Canadian Press. All rights reserved.