To answer your question about recommendations, you probably don't need to worry so much about them since you are from overseas. I know that USC has a competitive greek system, but I know a lot of women who go through with no recs at all and do just fine.
As for weather...well, bring lots of light clothes. It barely gets below 55 here and 45 is a real stretch. Nothing at all like the UK. You'd probably laugh at us all out here because a lot of Californians put on a sweater if it's colder than about 70.
