Cornwall, Ontario, will see a sunny start to the week before unsettled weather moves in, according to Environment Canada‘s forecast issued Monday, July 20, 2026. Monday will be sunny with a high of 26°C and a Humidex of 29, but showers and possible thunderstorms arrive Tuesday, with on-and-off rain chances through the week.
Tuesday, July 21, is expected to bring a mix of sun and cloud, becoming cloudy near noon with showers and a risk of an afternoon thunderstorm. The high will once again reach 26°C, with a Humidex of 31. For Wednesday, July 22, there is a 40% chance of showers and a cooler high of 20°C.
Conditions improve Thursday through Saturday, with mostly sunny skies Thursday and Friday before a mix of sun and cloud on Saturday. Highs will range from 24°C to 28°C. A 30% chance of showers returns Sunday with a high of 25°C. July normals for Cornwall are a high of 27°C, so temperatures will be near or below average — with Wednesday notably cooler.
The week reflects the changeable pattern forecasters have described for Ontario’s 2026 summer. Environment Canada’s new hybrid AI forecasting system, GEML, launched in May 2026, combines artificial intelligence with traditional physics-based models to improve accuracy for severe weather predictions and is now operational.
Residents planning outdoor activities should keep rain gear handy, particularly Tuesday and Wednesday, when the risk of showers and thunderstorms is highest.
