Weather in Porto in September. The average maximum daytime temperature in Porto in September is a warm 24°C (75°F). There are usually 8 hours of bright sunshine each day, which represents 60% of the 12 hours of daylight. The average sea temperature in and around Porto in September is 18°C (64°F). Expect 62mm (2.4 inches) of rainfall across the whole of September with 8 days with some rain falling. Maximum UV levels will be high (6 UV index) at midday when the skies are clear.