Southsea weather in May. The average maximum daytime temperature in Southsea in May is a comfortable 15°C (59°F), while the average minimum night-time temperature is a cool 8°C (46°F). You can expect on average 7 hours of sunshine a day in Southsea during May. 1  15. 15°C max day temperature. 2  7. 7 hours of sunshine per day. 3  12. 12 days with some rainfall. 4  8. 8°C min night temperature. 5  50. 50 mm of monthly rainfall.