Southern Ontario is bracing for an intense heatwave this weekend, as hot and humid air surges into the region. The combination of sweltering temperatures and building atmospheric instability is expected to bring a threat of thunderstorms, especially by Sunday, July 6.
Scorching heat and high humidity across southern Ontario
July didn’t hold back, ushering in southerly winds and a subtropical air mass that’s sending daytime highs soaring into the low to mid-30s°C (86–95°F). The humidex is forecast to reach or exceed 40°C (104°F) in many areas, with oppressive nighttime lows offering little to no relief.
On Sunday, Windsor is expected to feel like 42°C (108°F), while Toronto will see humidex values near 39°C (102°F). In Ottawa, it could feel as hot as 40°C (104°F), with overnight temperatures staying elevated across the region, limiting any chance of cooling off after sunset.
Heat warnings in effect across southern Ontario
Heat warnings are currently in place for Saturday, July 5, and Sunday, July 6, stretching across southern Ontario, including Toronto, the Highway 401 corridor, the Niagara region, Windsor, and Ottawa.
Vulnerable populations—including those with chronic health conditions, young children, and individuals without access to air conditioning—may face heightened risks as the apparent temperatures climb. The persistent overnight humidity will further strain the body’s ability to recover from daytime heat.
Storm threat rises Sunday with cold front arrival
Daytime heating and high moisture levels will create the instability needed for scattered thunderstorms on Saturday afternoon, with the severe weather risk peaking Sunday as a cold front sweeps across the region.
Strong to severe thunderstorms could develop along the Lake Huron shoreline, over southern Georgian Bay, eastern Ontario, and into western Quebec. These storms may bring large hail, damaging wind gusts, and torrential rain.
Further south, non-severe storms may still form across areas from Windsor to the Greater Toronto Area (GTA). The boundary and instability are expected to linger into Monday, bringing the potential for another round of active weather to start the workweek.
Stay alert as hot and stormy weather evolves
Keep an eye on radar imagery and be weather-aware throughout the weekend, especially as Sunday’s front approaches. While exact details on storm timing and location remain uncertain, the atmospheric conditions are primed for active and potentially severe weather in several parts of the province.
Hot, humid nights will continue to dominate the forecast, contributing to restless sleep and increasing the likelihood of heat-related illnesses if precautions aren’t taken.
Stay with The Weather Network for the latest updates on conditions across Ontario.
