May in Great Falls averages a daily high of 19.5°C with nights around 5.4°C. Expect roughly 10.8 wet days totalling 65.4mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Great Falls is August–September.
Based on GREAT FALLS 16ST, MT US (2.1km), 1995–2024. 14 years of May data.