Weather in Dec. can vary. I would pack a little of everything..some shorts, some pants, a light jacket with sweatshirts etc. The best is to pack things that can be layered. That way as it gets on to evening and the temps drop, all you need to do is put on another layer.
Keep checking out something like the long term outlook for the weather. Last year was cooler than it had been in the past.
But if you stick with the layering, you should be okay.