Residents of Sarnia, Ontario can expect a mix of sun and cloud with mild temperatures and several chances of showers through the week of September 13, 2026, according to Environment Canada.
Daytime highs are forecast to reach between 20°C and 26°C, which is above the usual mid-September maximum of 22°C. Tuesday, September 15 will see a high of 26°C with a 40 percent chance of showers during the day, and the risk of a thunderstorm in the evening. The wettest day is expected to be Thursday, September 17, with a 60 percent chance of showers and a high of 26°C.
The unsettled pattern is due to a stalled frontal boundary bringing humid air into southern Ontario, a setup The Weather Network says could lead to localized flooding if thunderstorms develop. Humidex values may reach the low 30s in the Sarnia area, with a Humidex of 32 forecast for Tuesday, making it feel warmer than the thermometer suggests.
No weather alerts are currently in effect for Sarnia. On Monday, September 14, skies were sunny at the Sarnia Chris Hadfield Airport, with a temperature of 18.9°C and light northeast winds. Sunrise that day was at 7:08 a.m. and sunset at 7:42 p.m.
Historically, the highest daily rainfall in Sarnia during September occurs around September 15, making this week’s showers typical for the season. Residents planning outdoor activities are advised to keep an umbrella handy, especially for Tuesday evening and Thursday.
