How Often Does It Rain in May? During May, Lisbon will feature about 17 mostly sunny, 4 partly sunny, and about 10 mostly cloudy days. You can also expect that 9-10 days will record some rainfall. However, only about 5 days record at least 1 mm (0.04 inches) or more, and only 2 days will see 10 mm (0.4 inches) or more.