Toronto's heat alert has been upgraded to extreme as Ontario and Quebec brace for a heat wave expected to push the mercury even higher than it has been.

David McKeown, Toronto’s medical officer of health, upgraded the alert early Monday. The extreme alert will be in effect until further notice.

The move comes as a blanket of hot and humid weather envelops much of the two provinces, stretching to the eastern U.S. seaboard, with temperatures expected to climb into the low- to mid-30 C temperatures this week across southern Ontario and Quebec. With the humidity, it will feel closer to the mid-40s.

Humidex and smog advisories are in place across southern Ontario from Windsor to Kingston and north to Parry Sound. A high heat and humidity warning also exists for southern Quebec.

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An official heat wave means three consecutive days with temperatures of 32 C or higher, which should arrive by Wednesday, said CBC meteorologist Johanna Wagstaffe. However, she said, relief should come by Friday, when a cold front arrives.

"This is the first real hot spell for southern Ontario and Quebec, and it's just getting started," said Wagstaffe. "It's basically a hot, humid and hazy week ahead."

A smog advisory for much of southern Ontario and Quebec was also still in effect Monday. Environment Canada issued the smog advisory Sunday for a wide area that includes Toronto, Hamilton, Barrie, Peterborough, London, Sarnia and Windsor.

During an extreme heat alert, residents are encouraged to check on family, friends and neighbours, especially isolated adults and seniors who are at greater risk of suffering from heat-related illness. Toronto also opened a number of cooling centres.

Other groups at risk include people with chronic and pre-existing illnesses, infants and young children, people on certain medications and those who are marginally housed or homeless.

Meanwhile, British Columbia is awaiting its own heat wave after less than ideal weather on the West Coast. Sun is in the forecast all week and by Wednesday Vancouver should get up to the low 30s.