Heat and humidity was the theme of the day across southern Ontario on Monday as a thick blanket of summer weather descends on the region, bringing muggy weather and threats of a humidex warning.

A mini-heat wave is expected to hit Toronto by Monday afternoon, when temperatures as high as 33 C are likely. The city has already declared a heat alert, warning residents to stay out of the sun whenever possible.

Toronto and the surrounding area have been warned to expect some of the hottest temperatures of the year. Almost the entire stretch of southern Ontario can expect temperatures above 30 C.

Environment Canada has issued a special weather statement telling regions including Windsor, Sarnia, London, Hamilton, Barrie and Peterborough to expect hot, muggy weather.

In Toronto, where the humidity will make it feel more like 38 C, the city's top medical officer issued a heat alert for Monday – warning residents to stay cool and drink plenty of water.

The public is encouraged to check up on family, friends and neighbours, specifically isolated seniors who are in greater risk of suffering heat-related illness.

Dr. Dave McKeown, Toronto's Medical Officer of Health, also suggests Torontonians:

  • drink lots of water, even before you feel thirst
  • cool down in air conditioned places such as shopping malls or libraries
  • take cool showers or use wet towels to cool down
  • wear loose, breathable clothing
  • avoid the sun and stay in the shade whenever possible

Toronto's hot Monday follows a warm weekend and an even-warmer Friday, when the afternoon high of 30.7 C set a new daily record.

Setting a new record for highest temperature on May 28 is not out of the question. The current record set in 1987 sits at 32.5 C, a fraction below Environment Canada's expectation for Monday.

Toronto also faces a chance of showers and thunderstorms early Monday morning and again overnight.

Thundershowers are also possible on Tuesday, which has a predicted daytime high of 28 C.