September in Arecibo averages a daily high of 30.5°C with nights around 20.2°C. Expect roughly 16.5 wet days totalling 260.8mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Arecibo is December–March.
Based on ARECIBO OBSERVATORY, PR US (14.2km), 1995–2024. 26 years of September data.