St. Thomas, Ontario is in for a warm and humid start to the week, with scattered showers possible, according to Environment Canada. As of 1:00 p.m. EDT on Monday, August 17, 2026, the city was mostly cloudy with a temperature of 24.6°C and a humidex reading of 30.
Forecasters say temperatures will peak near 26 or 27°C early in the week, with humidex values reaching 31 to 32 Monday and Tuesday, making it feel much warmer. Tuesday will bring a mix of sun and cloud, with a high of 27 and a humidex of 32. There is a 60 percent chance of showers Tuesday night, with a low of 19. Highs are expected to ease into the low-to-mid 20s by the weekend, with a 40 percent chance of showers Saturday and Sunday.
No weather alerts are currently in effect for the St. Thomas-Aylmer-Eastern Elgin County area. However, residents should be prepared for sudden changes in conditions as the week progresses.
Southwestern Ontario, including the St. Thomas region, is one of the most active areas for lightning in Canada. Summer humidity regularly pushes humidex values into the mid-30s or higher, making outdoor plans uncomfortable and potentially unsafe during stormy periods.
Climate normals for this time of year show a high of 25°C and a low of 15°C. Sunrise is at 6:33 a.m. and sunset at 8:24 p.m. Residents are encouraged to monitor local forecasts and plan accordingly if heading outside.
