September in Longmont averages a daily high of 26.9°C with nights around 7.8°C. Expect roughly 6.5 wet days totalling 32.3mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Longmont is September–October.
Based on LONGMONT 2 ESE, CO US (2.1km), 1995–2024. 18 years of September data.