The heat warning should end today

A person with an umbrella walks through the ByWard Market during a rainshower. Aug. 13, 2025. (Ted Raymond/CTV News Ottawa)

A multi-day heatwave in Ottawa is coming to an end as a cooler, less humid airmass moves into the region.

The ending could be dramatic, as cool air and warm air mixing can create thunderstorms.

In the meantime, temperatures will still be well above normal for most of the day Wednesday, with a high of 30 C forecast. The humidex could reach 38.

There is a 70 per cent chance of showers in the morning followed by a 30 per cent chance of showers in the afternoon with a thunderstorm risk.

The temperature was already 23C with a humidex of 34 at 6 a.m.

The normal high for this time of year is around 25 C and the normal low is 15 C.

A thunderstorm could be heard over downtown Ottawa just before 9 a.m. as a rainy system moved across the region. The temperature dropped four degrees between 8 and 9 a.m., according to Environment Canada.

Thursday’s forecast is sunny with a high of 28 C and a humidex of 32.

Friday could see more sunshine with a high of 29 C.

Saturday’s outlook is hot, with a lot of sunshine. Some clouds could move in on Sunday.

Hottest day of the year

Tuesday had the highest temperature of 2025, officially reaching 35.3 C, just barely beating the previous high mark for the year of 35.2 C on June 23.

June 23 still holds the year’s highest mean temperature—the average of the daily high and low—at 27.7 C. Tuesday’s mean temperature was lower, at 25.9 C, because the low was 16.5 C.

There have been 21 days so far this year with high temperatures above 30 C, including six straight days in August, according to Environment and Climate Change Canada.

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