I was going to answer you, then thought it was a dumb answer, but now no one is answering you. So maybe my dumb answer will be OK.
A sweatshirt will likely be fine.
BUT I live in Western WA, and I always want to scoff at the concept of it being "cold" in southern CA. I grew up in San Jose (officially northern CA but really somewhere in the middle, LOL) and when I lived there I felt that 50 degrees was just beyond cold, and I had to get out of that mindset when I moved to WA. Now 50 is shorts weather.
But you know what? At night, even when I'm visiting my brother in San Diego, southern CA can get cold! I don't know if it actually is a desert, or desert conditions, but it seems like it, with a BIG difference between day and night temperatures, and it really seems to bother my system. Even though I don't think it should.
So a sweatshirt should do you fine (with a rain poncho on top if it's raining), but don't be freaked out if you find yourself shivering at some point.
So that's my dumb answer. Dumb b/c it'll probably turn out that you're from Minnesota or northern Canada or somewhere just horribly snowy, and you will be boiling your whole trip and think I'm a big wimp.
