Temperatures are expected to exceed 30 C - with the humidex in the mid-40s - throughout the Canada Day long weekend and beyond.
It's going to be a hot week all across Southern Ontario.
Daytime highs are forecasted to reach the low to mid thirties with humidex values into the mid forties.
"This will be the most significant heat event in the past few years", said the national forecaster. Find a cool space, a body of water to swim in, drink lots of liquids and wear your linen, if you've got it, he said.
"Uncertainty about when will the heat wave still persists".
Environment Canada also said that overnight low temperatures will only fall to the low-to-mid-20s, which means there will be no relief from the heat.
In Ottawa, organizers are watching the weather to decide if the annual Canada Day show will take place on Sunday, Canada Day, July 1st.
Maxime Roy, from the city's public health department, recommended that people avoid physical activity outdoors and stay hydrated.
Heatwave response: Families are being advised to take special care of the youngest and oldest members of the family during the next five straight days of heat and humidity Environment Canada is predicting.
The Perth District Health Unit has also issued an extended Extreme Heat Warning for the area.
It says "a very warm and humid airmass" will settle over eastern Quebec and the Maritime provinces this weekend, and will last into next week.
Environment Canada advises that there is still uncertainty regarding when the heat event will come to an end, as current indications suggest the heat could continue into the middle of next week.