Ontario Faces Heat Dome, Severe Storms, and Weather Warnings Ahead of Canada Day

A significant heat event is forecast to affect southern Ontario in the lead-up to Canada Day, as a heat dome forming over the eastern United States extends its influence northward. This atmospheric ridge is expected to bring very hot and humid conditions to the region, with temperatures climbing well into the 30s Celsius and humidex values projected to reach the 40s. The event is anticipated to peak around Canada Day, with Toronto-Pearson Airport forecasting a high of 34°C and a feels-like value of 45. This could challenge the site’s record for the hottest Canada Day.

The heat dome, centred over the eastern half of the U.S., is expected to persist through at least the middle of the week. Its northern edge will encompass the Great Lakes region, beginning Sunday, with the most intense conditions arriving midweek. Winds circulating around the high-pressure centre will funnel hot, muggy air into southern Ontario, particularly impacting communities away from the moderating effect of Lake Ontario.

The Weather Network notes that this setup is also conducive to the formation of organized clusters of thunderstorms, which may circulate around the ridge of high pressure. The exact timing and intensity of these storms remains uncertain, but the potential for severe weather persists during the heat event.

Extreme heat is recognized as a leading cause of weather-related fatalities worldwide. Health authorities and meteorological agencies advise precautions including frequent breaks, adequate hydration, and access to air conditioning to mitigate the risk of heat-related illness. Vulnerable populations, such as the elderly, those with chronic health conditions, and individuals without air conditioning, are at heightened risk. Certain medications can further reduce the body’s ability to tolerate heat.

In addition to the impending heat, southern Ontario has already experienced severe weather this week. Thunderstorms swept through the region overnight, bringing intense lightning activity to downtown Toronto. A video posted on social media captured a lightning strike appearing to hit the Gardiner Expressway. According to Environment Canada meteorologist Simon Legault, Toronto-area weather stations recorded between 10 and 12 millimetres of rain during the storms, with lightning strikes concentrated between 11 p.m. and 1 a.m.

Environment Canada cautions that lightning kills two to three Canadians and injures approximately 180 others annually. The agency reiterates that if thunder can be heard, individuals are within striking distance and should seek shelter.

Southwest of London, Ontario, a tornado touched down west of Glencoe on Thursday evening. The Northern Tornadoes Project (NTP), based at Western University, confirmed the tornado, which left a narrow path of damage through rural Middlesex County. The tornado was preliminarily rated EF0, the lowest rating on the Enhanced Fujita scale, which ranges from EF0 to EF5. Wind speeds with an EF0 tornado can reach up to 130 kilometres per hour.

NTP director David Sills stated that damage was observed to numerous trees, at least one farm, and an overturned recreational vehicle. Large hail also accompanied the storm as it moved southeast from Sarnia toward London. Environment Canada received reports of hail up to three centimetres in diameter in the affected area, along with damage to trees and farm structures.

Amanda Vanderkuyl, a resident south of Glencoe, described witnessing the storm’s development from her horse farm. She reported seeing rotation in the clouds before the tornado formed, and described the storm as arriving like a 'freight train,' bringing rain, hail, and strong winds. Her property sustained only minor damage, such as toppled plants.

The Glencoe tornado is the fourth confirmed or preliminarily confirmed tornado in the broader London area since early May, following events near Lucan, south London, and Melbourne. NTP survey teams planned to return to the area to assess the final rating, measure the path length and width, and determine if the rating should be upgraded.

In the aftermath of the Thursday storm, visible damage was reported near the intersection of Concession Drive and Old Airport Road in Glencoe. The NTP’s ongoing assessments are expected to provide further details on the tornado’s impacts.

Meanwhile, a weather advisory was issued for several parts of southern Ontario on Friday morning due to dense fog. Environment Canada reported near-zero visibility in areas including Hamilton, St. Catharines, Grimsby, the northern Niagara region, Brantford, and the County of Brant. Drivers were warned of hazardous travel conditions, urged to slow down, use low-beam headlights, maintain extra distance between vehicles, and allow additional travel time. The fog was expected to dissipate later in the morning, improving driving conditions.

Looking ahead, meteorologists expect calmer weather over the weekend in Toronto, with temperatures rising to near 30°C at the start of the new week in advance of the heat dome’s full impact. The convergence of hazards—extreme heat, severe thunderstorms, tornadoes, and impaired visibility—highlights the range of weather challenges facing southern Ontario as Canada Day approaches.

Authorities continue to monitor the evolving conditions and advise the public to stay informed of weather alerts and take recommended safety precautions.