Hi Patrick,
Would help to know if you will need dress or casual clothes during your trip. Also, will you be outside a lot or mostly just between home/work/establishments?
My general suggestion would be "layers". I'm thinking you probably have a light jacket or could make use of a light fleece pullover (casual) or raincoat (more formal) in Southern Cal. A couple of long sleeve T-shirts or "thermal look" shirts can be worn under light shirts/sweaters/pullovers. Even two worn together is kinda the style and the air between layers traps heat. I would also suggest a few cotton sweaters or vests -- adaptable to cool nights in So. Cal, but again can be layered for warmth. I have a raincoat with a zipout lining that can double pretty well as a more formal winter coat or light fall/spring coat. If you're going to be trekking, get yourself a couple of pairs of insulated socks. You can convert light loose-fit pants to cold weather gear with "long johns" -- course us women folk (at least me) sometimes do this with pantyhose but ... Look for stuff with Thinsulate -- as long as you aren't sporting pink and powder blue linen, I'm sure you have lots of stuff in your current wardrobe that can be "winterized" with the right undergarments. Make sure to have gloves and either a scarf, hat, hood, muffs, or whatever you feel most comfortable sporting to keep your head, and particularly neck & ears, warm.
Of course, my hubby would probably suggest packing shorts just in case the mercury spikes up over 50 

Evie 