May in Waterloo averages a daily high of 22.1°C with nights around 9.4°C. Expect roughly 12.7 wet days totalling 119.2mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Waterloo is August–September.
Based on WATERLOO MUNICIPAL AIRPORT, IA US (8.4km), 1995–2024. 30 years of May data.