Meteo Hagåtña in maggio

Guam forum regulars, including at least one longtime resident, describe September rain as more of a nuisance than a trip-wrecker: most showers last just a few minutes, often happen at night, and the handful of afternoon ones that do occur usually blow through within minutes. That's despite September sitting in the wettest, most typhoon-exposed stretch of the year (July through November). The heat and humidity are the bigger factor day to day - it rarely drops much below 75F even overnight - so most visitors plan around the mugginess rather than the rain itself.

Forum posters place March within Guam's dry season (December-March), well clear of the main rainy stretch that runs roughly late June through October/November. Typical temperatures run mid-80sF, and even on days that do bring rain, posters say it tends to hold off until evening, leaving daytime hours mostly sunny for beach time and sightseeing. December through March is consistently named the best overall window to visit, prized specifically for avoiding both the heavier rain and the typhoon risk that builds through the second half of the year.