May in Portland averages a daily high of 17.8°C with nights around 7.2°C. Expect roughly 12 wet days totalling 91.6mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Portland is June–September.
Based on PORTLAND JETPORT, ME US (4km), 1995–2024. 30 years of May data.