September in Lahti averages a daily high of 15.1°C with nights around 7.3°C. Expect roughly 11.2 wet days totalling 52.7mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Lahti is June–August.
Based on HEINOLA ASEMANTAUS, FI (32km), 1995–2024. 30 years of September data.