I have to respectfully disagree with MrsJackSparrow4Life and point out that December in SoCal is not always warm - especially depending on the individual person, what they are used to and what feels warm or cold to them. We are very lucky if the days are 70 degrees, but there have also been years where it has been pouring rain (like bad, heavy El Nino rain!) through much of December. Also the days can be in the high 50's or low 60's (like they were last year when we were there) while the nights can drop down to the 30's or 40's and it is very cold. Last year, I mistakenly left my coat and mittens in the hotel room one night and was wandering around in a t-shirt in 40-degree weather and I was freezing. I personally am always warm, and it takes a
lot for me to get cold, but that is just me. I can see how the temperature changes affect other people. Some of my friends get cold very easily, even at 60 degrees. So I would not count on coming out here in December and having 70 degree weather the entire time. It
may be that way, but if you get chilled easily, then make sure you are prepared. Of course, it always feels a little warmer in DLR during the daytime than what the actual temperature might be. We are supposed to be coming up on another El Nino so that may bring lots of rain again.
It's funny what MrsJackSparrow4Life said about the locals running around in cold weather gear in 70 degrees while others are in shorts.

I don't drag out my coat and gloves until it gets down to 40 degrees, but other friends of mine have the gloves and neck scarf out when it is 60 degrees (and we are lifelong SoCal residents) and I giggle at them. We are just different. And whenever we see people walking around in shorts or summer wear in 40 or 50 degree-weather at DLR (and we have seen them), we always say to each other, "I bet they don't live in California." I don't know how often we have been right, but we always assume the folks wearing warm weather clothing in December are tourists.