Severe weather risk for Southern and Eastern Ontario on Wednesday and Thursday
Ottawa, Wednesday June 4, 2025, 08:00 a.m. (local time) – Environment Canada is forecasting a notable outbreak of severe thunderstorms across Southern Ontario this Wednesday, with a moderate risk level (2 out of 4). Residents should prepare for damaging wind gusts up to 90 km/h (56 mph), toonie-sized hail (2–3 cm) and heavy rainfall with potential flash flooding up to 50 mm (2 in).
Wednesday morning: early storms in Northeastern Ontario
The first wave of thunderstorm activity will begin overnight Tuesday into early Wednesday, mainly over Northeastern Ontario. Areas likely to be impacted include north of Sault Ste. Marie, Timmins, Moosonee, and regions north of Lake Superior near the Quebec border.
These initial storms are expected to be non-severe, but they may still feature isolated lightning and moderate rainfall totals, with localized amounts reaching 30 mm and peak hourly intensities near 15 mm. Confidence in this early setup is currently moderate.
Wednesday afternoon and evening: strong thunderstorms over Southern Ontario
From midday into the evening, Southern Ontario becomes the focus for potentially dangerous thunderstorms. The most intense cells could bring a combination of high winds, large hail, and heavy rain. Affected areas may include cities and towns throughout the Greater Toronto Area, Niagara Region, London, and Kitchener-Waterloo.
Hazards expected include:
- Wind gusts up to 90 km/h (56 mph), strong enough to topple branches, damage weaker structures, and scatter loose objects
- Hail up to 3 cm in diameter, capable of damaging crops, rooftops and vehicles
- Localized downpours reaching 50 mm, particularly where repeated storms pass over the same region
- Flash flooding and pooling on roadways, especially in urban areas or poorly drained zones
Environment Canada notes that smoke in the upper atmosphere may suppress storm development, adding uncertainty to the forecast. Despite the moderate-to-high impact potential, this factor reduces overall confidence regarding storm severity.
Thursday: Eastern Ontario under threat
On Thursday June 5, the risk of thunderstorm activity continues but shifts eastward, with Eastern Ontario—including Ottawa, Cornwall, Brockville, Perth, and east of Kingston—seeing a slightly elevated risk.
Forecasts suggest non-severe thunderstorms are again possible across southern and far northern Ontario, but ‘Area A’ in Eastern Ontario could see isolated stronger storms with:
- Wind gusts of 70–90 km/h (43–56 mph)
- Nickel-sized hail (up to 2 cm)
- Localized rainfall totals of 15 mm, with potential for brief heavy showers
These storms may again be scattered in nature, with plants, crops, and unsecured outdoor items at risk. Although the probability remains moderate, Environment Canada warns that some convective cells could strengthen significantly.
Important notes on preparedness
Environment Canada’s alerts can evolve rapidly. This week’s setup—with heat, instability and atmospheric moisture—marks a transition into a more active thunderstorm pattern for Ontario. Wednesday’s storms in particular require careful monitoring, especially in Southern Ontario, where afternoon and evening timing overlaps with peak commute hours.
Keep an eye on updates from local weather apps, including Instant Weather, and remember the lightning safety mantra: “When thunder roars, go indoors.”
If you are in Ottawa, Toronto, Hamilton, or Windsor, make sure to secure outdoor items like patio furniture, garbage bins and toys, as they can easily become hazards during strong wind gusts. Drivers should also prepare for reduced visibility, sudden downpours, and water accumulation on roads, especially in low-lying or poorly drained areas.
Stay weather-aware this week across Ontario, and keep monitoring alerts as the situation develops.
