May in Thornton averages a daily high of 22.4°C with nights around 6.6°C. Expect roughly 9.8 wet days totalling 63.3mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Thornton is September–October.
Based on NORTHGLENN, CO US (4.9km), 1995–2024. 29 years of May data.