Hi there,
I was there at that exact time in 2010... wow- nearly 2 years ago!
When we arrived, the weather was extremely wet and cold for the first 2 days. However, all of the locals and CM's kept saying "this is the worst it's been in years" and "it's so unexpected".
After a few days, it dried out in time for Christmas Day. While the sun was out, I do remember (and all of our photos show me) that we were all still in long sleeves and long pants. At night we needed beanies and scarves.
I know everyone will say it can't be predicted, but I would plan for wearing layers to cover all bases.
Happy planning and have a great trip in December!