September in Sarnia averages a daily high of 23.4°C with nights around 13.8°C. Expect roughly 9.9 wet days totalling 85.3mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Sarnia is May–September.
Based on PORT HURON, MI US (1.3km), 1995–2024. 30 years of September data.