Consistent daytime high temperatures exceeding 30 C (86 F) are predicted.
A cold front is forecast to pass through southern Ontario this afternoon ushering in a slightly cooler and less humid air mass.
Twelve deaths were reported in Montreal and five were reported in Eastern Townships.
Kaiser said one reason Quebec has been reporting so many deaths is because it's part of the province's extreme heat plan, where health officials work in tandem with first responders and emergency rooms to track down potential cases of heat-related illnesses and deaths.
Dr. Mylene Drouin, head of public health for the Montreal region, said the number of fatalities has risen since authorities first announced six deaths on Tuesday, but that the spike is not unexpected.
An emergency coordination centre has been activated to supervise those measures and Montreal Mayor Valerie Plante has called on Montrealers to show solidarity with at-risk people.
Couillard noted that the weather is expected to cool down by Friday but "we expect to have this kind of episode every year". The person had complained of the heat and breathing difficulties and died after paramedics had arrived.
"Usually in a normal situation we have 10 deaths per day, so we're not in an excess of mortality", Drouin said. A high of 26 and low of 14 is expected in Toronto on Friday, Ms. Hobelman said, with a chance of showers and a thunderstorm.
Urgences-sante says it has been inundated by the volume of calls since a heat wave enveloping Eastern and Central Canada settled in for an extended stay over the weekend. However, the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.