Normally, my 0 degree is actually too warm, but it's good to have for when it's really chilly out. I think you'd be fine with a 0.
There are lot's of tricks if it gets super cold to "stretch" your 0 degree bag (pardon me if you already know these):
Use two sleeping pads, one on top of the other.
Wear double long johns, socks, a hat, and gloves to bed.
Put a hot water bottle (Nalgene with water as hot as you can stand it), some say by the groin (good luck sleeping), others say by the feet.
Get a sleeping bag liner.
Keep (and eat) high protein snacks in the bag with you.
I'm sure you can think of other "stay warm" ideas. All that to say: a 0 degree bag should do you fine for Southern California and probably much of Northern California too.
Just to be safe, always keep an eye on the weather. If it looks really nasty, grab that Yukon bag (or reschedule)

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