September in Tri-Cities averages a daily high of 27.3°C with nights around 9°C. Expect roughly 3.2 wet days totalling 7.3mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Tri-Cities is April–May.
Based on PASCO TRI CITIES AIRPORT, WA US (6.6km), 1995–2024. 27 years of September data.