Widespread heat wave set to scorch Ontario and Atlantic Canada
A hot and humid air mass is on its way, replacing this week's cooler weather with a stretch of 30°C+ days, steamy nights and very little relief in Eastern Canada.
A major pattern shift will send Eastern Canada into a prolonged period of heat and humidity, beginning this weekend and lasting well into next week.
Temperatures across Ontario, Quebec and parts of Atlantic Canada are expected to rise into the 30s, with humidex values reaching the 40s in some areas.
SEE ALSO: ACHOO! The science behind the solar sneeze
Humidity ramps up across 2,500-km stretch
By Friday, southwesterly winds will signal the arrival of much warmer air, with high humidity extending more than 2,500 kilometres from Ontario to Newfoundland by Sunday.
The change occurs as a ridge forms, transforming the area from comfortable to almost oppressive overnight.
Conditions will be particularly muggy in the Greater Toronto Area (GTA), with overnight lows in the low- to mid-20s, providing little, cooling relief.
Heat could last for days
Daytime highs are expected to return to the 30s in much of southern Ontario, the St. Lawrence Valley and even parts of Atlantic Canada, including New Brunswick and eastern Newfoundland.
Cities such as Windsor, Toronto, Ottawa, Montreal and Fredericton could experience five or more days of temperatures above 30°C.
SEE: Look up! Perseids and a Planet Parade shine in August 2025
Between Friday and Wednesday, humidity will cause the humidex to rise into the high 30s, possibly extending into the low 40s.
According to Environment and Climate Change Canada (ECCC), humidex values above 40 occur on only one day per year in Ottawa. The upcoming stretch may result in two.
August in Toronto averages just more than six days with a humidex of 35 or higher. That threshold could be met in less than a week.
Stay with The Weather Network for all the latest across Ontario, Quebec and Atlantic Canada.