September in Mount Prospect averages a daily high of 24.3°C with nights around 13.5°C. Expect roughly 9.2 wet days totalling 72.5mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Mount Prospect is May–September.
Based on CHICAGO PALWAUKEE AIRPORT, IL US (6.6km), 1995–2024. 28 years of September data.