Heatwave expected to blanket southern Ontario this week

Southern Ontario is heading into a prolonged stretch of hot and increasingly humid weather beginning Monday, with temperatures expected to peak midweek and persist through the Canada Day holiday and into the start of July.

Environment Canada forecast guidance shows daytime highs climbing steadily across the Greater Toronto and Hamilton Area, including Mississauga, Toronto, Brampton, Hamilton, Oshawa and Pickering, with a multi-day heat event setting up from June 29 through at least July 4.

Monday (June 29) is expected to be the starting point of the hot stretch, with daytime highs around 28°C in Toronto, Mississauga and Brampton. Humidity will already begin to build, pushing humidex values into the low 30s, making it feel noticeably warmer than the thermometer suggests.

Overnight lows near 20°C will also keep conditions warm and humid, limiting relief after sunset.

The core of the heatwave arrives Tuesday and Wednesday, when daytime highs surge into the low to mid-30s across much of the region.

• Toronto: near 33°C
• Mississauga / Brampton: around 35°C
• Hamilton: similar mid-30s inland heat
• Oshawa / Pickering: 32°C, slightly moderated near Lake Ontario
• Niagara Region: 32°C, with humidity pushing it higher

With humidity factored in, humidex values are expected to reach the upper 30s and possibly near 40, especially in urban inland areas like Brampton and parts of the GTA.

These conditions meet the criteria for extreme heat risk, particularly where overnight lows remain around 22°C to 24°C, preventing significant cooling.

On Canada Day (Wednesday, July 1), southern Ontario is expected to remain in the grip of the heat, with highs near 33°C to 35°C across much of the GTA and surrounding regions.

Humidity will again make it feel closer to the upper 30s, and conditions may feel oppressive during outdoor events, barbecues and fireworks gatherings.

There is also a chance of scattered showers or thunderstorms on Canada Day developing during the afternoon and evening, which is typical during high-heat, high-instability setups. Any storms that do develop could bring brief heavy downpours, gusty winds and small hail, though widespread severe weather is not guaranteed at this stage.

From Thursday through Friday, the heat is expected to continue with highs generally in the 32°C to 35°C range, though some variability is possible depending on cloud cover and thunderstorm activity.

Periodic 30 to 40% chances of showers or thunderstorms remain in the forecast through the latter half of the week. Localized storms could briefly break the heat but may also increase humidity in their aftermath.

Even at night, temperatures are expected to remain warm, generally between 20°C and 24°C, meaning limited overnight relief, especially in urban areas where heat retention is strongest.

Health officials typically advise residents to take precautions during prolonged heat events:

• Seek air-conditioned spaces such as libraries, malls, or cooling centres
• Limit strenuous outdoor activity during peak afternoon heat
• Stay hydrated and check on vulnerable neighbours
• Use fans carefully—ineffective when humidex climbs into the high 30s
• Never leave children or pets in parked vehicles