Heat warning issued with early blast of summer about to hit GTA

Daytime highs expected to reach 30 C with humidex making it feel like 36 C in Toronto: Environment Canada

Published May 17, 2026  •  2 minute read

Patrons enjoy the patio at Mill St. Brewery's Ottawa Brewpub near Parliament Hill during the warm spring weather on Sunday, May 17, 2026. Photo by Ashley Fraser /Postmedia Network

The unofficial start of summer will coincide with a blast of summerlike heat across southern Ontario.

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Yellow heat warnings have been issued by Environment Canada for a region that stretches from Windsor to Markham and Niagara Falls to Barrie starting Victoria Day Monday and stretching to Tuesday night.

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In Toronto, daytime highs are expected to reach 30 C with the humidex making it feel like 36 C before temperatures drop to a less-than-seasonable 16 C on Wednesday. A high of 19 C and low of 10 C are typical for this time of year.

Showers are also possible throughout the region on both days as a warm air mass moves north from the United States.

Watch for early warning signs

Environment Canada said people should enjoy the weather cautiously as “extreme heat can affect everyone’s health.

“Determine if you or others around you are at greater risk of heat illness,” an Environment Canada warning said. “Check on older adults, those living alone and other at-risk people in person or on the phone multiple times a day.

“Watch for the early signs of heat exhaustion in yourself and others. Signs may include headache, nausea, dizziness, thirst, dark urine and intense fatigue. Stop your activity and drink water.”

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Temperature jump may feel ‘dramatic’ to some

Much of Ontario was already enjoying a blast of summerlike heat on Sunday with Windsor being the hottest place in the country at 28.5 C, according to Environment Canada. That was on a day when Cape Stallworthy, Nunuvut, about as Far North as Canada gets, had a high temperature of -10.7 C.

While temperatures like that are well in the past for the GTA, Environment Canada meterologist Kristina Kretchman told The Canadian Press that the jump from below-normal spring warmth to mid-summer conditions may feel “dramatic” to some.

To beat the heat, Environment Canada advised drinking lots of water; closing blinds or shades to keep out sunlight; using air conditioners or fans; or taking advantage of cooling centres, community centres, libraries and shaded parks.

“Plan and schedule outdoor activities during the coolest parts of the day,” Environment Canada said. “Limit direct exposure to the sun and heat. Wear lightweight, light-coloured, loose-fitting clothing and a wide-brimmed hat.”

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