An Environment Canada warning about the heat is in effect for Southern Quebec including Quebec City, Montreal, Laval, Gatineau and the Eastern Townships.
Parts of south central Saskatchewan, including Saskatoon, Regina and Moose Jaw, were under tornado watches and warnings Monday afternoon.
Peak humidex levels in the low 40s are expected for the next several days with another moisture-laden air mass expected to move in, according to Environment Canada.
Meanwhile, southern Ontario was blanketed by oppressive heat and humidity. By mid-afternoon Monday in Toronto and Ottawa temperatures hit 31 and felt like 39 with humidity.
That combination could be particularly hard on the elderly and people with chronic respiratory and heart conditions, Hull said. "Avoid strenuous exercise and stay well hydrated."
Why is it so hot?
July and August tend to be our hottest and most humid time of the year, said Hull. Record high temperatures were set Monday in Quebec and Atlantic Canada. In Halifax, the temperature hit 32 C. Quebec City and Badger, N.L., also hit record temperatures, cracking 31 and in Bathurst, NB it was a scorching 35.8.
"I wouldn't be surprised to see more high temperature records broken over the next couple of days," said Hull.
A ridge of high pressure spinning off the U.S. east coast is behind the hot, sticky air mass stretching from Northern Ontario to Newfoundland.
"High pressure often means sinking air and when that air sinks it heats up even more," said Hull.
Environment Canada has put a large swath of Eastern Canada under a humidex advisory or high heat and humidity warning as temperatures in many areas across Ontario, Quebec and parts of the Maritimes will feel humidex values close to 40 or above.
Definitions as to what constitutes a heat wave differ by province but in Ontario, it's three days of temperatures above 32.2 C(before factoring in the humidex).
This ridge is going to take its time to clear out, so for some areas of Central Canada there may not be any relief until later this week.
Humidex values are expected to fall below 40 in Southern Ontario and Quebec by Friday.
That relief will come in the form of a cold front and we will have to be on the lookout for some strong storms associated with this cooler air mass clashing with the humid, hot one in place now, said Hull.
A extreme difference in air masses over Alberta and Saskatchewan — where cooler air from the west is clashing with hot air pushed from the east — could "spark up" some severe thunderstorms over the Prairies, Hull said.
A tornado watch is in place for Saskatoon and Regina, where a severe weather outbreak in Central and Southern Saskatchewan is possible today.
"All the right ingredients are in place" including a moist, unstable air mass coming in from the U.S. and a dry line associated with a low pressure system moving in from Alberta, said Hull. "When those two air masses collide it could make for an extremely dangerous situation."