September in Downtown DC averages a daily high of 27.6°C with nights around 16°C. Expect roughly 9.4 wet days totalling 105.8mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Downtown DC is April–May.
Based on NATIONAL ARBORETUM DC, MD US (4.8km), 1995–2024. 25 years of September data.