Hi there
we have some onshore winds during all time of the year, but not very often. Only a few beaches get affected by those winds (well i'm speaking for the Lisbon area). Guincho beach is well know for being windy most of the days, so it's good if you're into windsurf or kitesurf.
However, Carcavelos beach is not that windy and very popular. Sadly, has no good waves during summer time. Further north, you have the region of Ericeira, which has some nice spots for you to surf, and it's actually my sugestion for you.
I hope i could help, and if you come here for the summer, give me a ring and i'll gladly surf with you (although i'm a newbie

)