September in Allentown averages a daily high of 24.8°C with nights around 12.6°C. Expect roughly 9.5 wet days totalling 122.8mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Allentown is May–June.
Based on ALLENTOWN LEHIGH VALLEY INTERNATIONAL AIRPORT, PA US (5.8km), 1995–2024. 30 years of September data.