"Severe thunderstorm watches are already out from Manitoulin Island east all the way to Quebec City, and we'll keep an eye out for watches to start being issued further south for lower Great Lakes later today," CBC meteorologist Jay Scotland said on Friday.
Heavy rain, hail, strong wind and even a possible tornado are among the threats ahead of the cold front.
Toronto and Ottawa both see the risk for severe thunderstorms today with highs of 34 C that will feel over 40 with the humidex.
Montreal also sees the risk for severe weather today with a high of 33 C, feeling like 45 with humidex.
Heavy rain in storms could mean 40 millimetres in southern Ontario and as much as 60 mm in western and southern Quebec, with higher amounts possible in localized areas.
The Toronto area reached official heat wave status — three straight days of 32 C or higher — when a high of 33.7 C was measured at Pearson International Airport Thursday, and Ottawa was officially under a heat wave by Thursday.
In Mississauga, hundreds of residents of an apartment building were forced outside in the sweltering conditions early Friday after an electrical fire cut power to their building.
The Maritimes will also be muggy and unsettled Friday as humid air moves in with humidex values in the low to mid 30s, and a thunderstorm risk for New Brunswick.
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