September in Janesville averages a daily high of 24.4°C with nights around 10.7°C. Expect roughly 9.3 wet days totalling 88.7mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Janesville is May–September.
Based on AFTON JANESVILLE WWTP, WI US (6.3km), 1995–2024. 30 years of September data.