Environment Canada heat warnings cover almost all of eastern Ontario and western Quebec Wednesday.
Ottawa reached a high of 30 C late Tuesday afternoon, with the air feeling like 36 around then. Wednesday's forecast is for a similar high that could feel like 40 C in the afternoon at its worst.
It already felt like 37 at the airport at noon.
There's also a slight chance of afternoon and evening showers. That includes the risk of a thunderstorm.
Forecasted highs for Ottawa the rest of the week are a few degrees below this, starting with 27 C Thursday feeling as muggy as 34 C, then an overnight low of 18 C.
People should look for shade, cool water or air conditioning when it's this hot and watch for signs of heat illness, including rashes, cramps and fainting.
There are a few exceptions to the heat warnings, such as the Deep River and Maniwaki areas.
Heat warnings are issued for southern Ontario, including the Ottawa region, when there are either two days in a row with forecasted highs of at least 31 C and lows of at least 20 C, or two days in a row with humidity expecting to make it feel like 40.
Quebec heat warnings are issued when the temperature hits 30 C and humidex is forecast at or above 40.