St. Thomas, Ontario, will see a sunny and warm start to the week before unsettled weather moves in, according to Environment Canada. No weather alerts are in effect as of Monday, July 20, 2026, but residents should prepare for changing conditions by Tuesday.
Monday brings a mix of sun and cloud with a high of 26°C and a humidex of 31. The UV index will reach 9, which is considered very high. Winds will become south at 20 km/h in the afternoon. People spending time outside should wear sunscreen and seek shade during peak hours.
Tuesday, July 21, will see showers along with a risk of thunderstorms. Winds are expected to pick up as a cold front passes through. They will start from the southwest at 30 km/h with gusts to 50 km/h, then shift to the west at 50 km/h with gusts reaching 70 km/h in the afternoon. The high will be 24°C, but the humidex will make it feel more like 32.
Environment Canada reminds residents that lightning can strike up to 16 kilometres from a thunderstorm. When thunder roars, the safest place is indoors. The St. Thomas area sits within a region that often sees severe summer storms because warm, humid air collides with cold fronts moving across the Great Lakes.
Wednesday will bring a mix of sun and cloud with a 30 percent chance of showers and a high of 21°C. Looking ahead, Thursday and Friday will bring sunny skies with highs of 25°C to 26°C, while Saturday will be a mix of sun and cloud with a high of 27°C. Sunday brings another chance of showers with a high of 28°C. The week’s temperatures are close to the normal July high of 26°C for nearby London.
