Duration, not intensity, is the danger in Ontario’s latest heat wave: meteorologist

Millions of Canadians are facing yellow or orange-level heat warnings as temperatures soar to the mid-30’s. John Vennavally-Rao reports.

TORONTO – If you’re sweltering in southern Ontario right now, you might be hoping the heat is at least going to break a record. But it turns out misery doesn’t even come with bragging rights this time.

Peter Kimbell, a warning preparedness meteorologist with Environment and Climate Change Canada, has checked the numbers — and despite an orange heat warning forecasting highs of 34 C to 37 C, most places will fall short of breaking daily records this week.

Windsor is the exception: a forecast high of 37 C on Canada Day could put it over the top.

But it’s the duration of this heat wave, not a single scorching day, that has Kimbell concerned.

“It’s going to be pretty long duration. We have four consecutive days,” Kimbell said. “That’s the really big issue.”

That duration is why the warning was upgraded to orange. Canadians are still getting used to the colour-coded system, introduced by Environment and Climate Change Canada late last year. Orange alerts are meant to be uncommon, issued only when weather conditions may cause significant damage, disruption or health impacts.

“When the duration exceeds three days, we can consider if the impact is sufficient to go to an orange warning, and so that’s what we’re expecting,” Kimbell said. “Heat that really can be significant for people everywhere, and it can be dangerous for people who are most vulnerable.”

With humidity, it will feel closer to 44 C in some areas — and there will be little overnight relief. For two nights in a row, the low in Toronto may only go down to just 24 C, eight degrees above normal.

Kimbell is urging people to check on those most at risk.

“If you know people who are in vulnerable situations, check up on them. I think they really need support when it’s so hot,” he said.

Simply put, he said, it will be “hot and uncomfortable.” The cause: a ridge of high pressure over the southern and central U.S. is funnelling hot, humid air across the Great Lakes and into Ontario.

As for how unusual heat waves of this duration are, Kimbell said they happen often enough.

“Every couple of years we get a stretch of at least three days or so with temperatures above 34 or 35,” Kimbell said.

The most extreme heat wave on record in Ontario happened nearly 90 years ago, before the humidex existed. Kimbell said Toronto hit 40 C for three straight days in July 1936, a stretch that killed more than 200 people.

“It was not pretty.”

This current wave doesn’t come close to that — but it may not be over after this week. Kimbell said models suggest another heat warning could follow.

“I wouldn’t be surprised if the heat warning reappears next week,” he said. “So that makes it even more challenging.”