I was there the first week of December 1997, and boy did the weather fluctuate!
Here's a brief synopsis:
Day 1, in the 80's, rainy most of the day
Days 2-5, beautiful and clear, but COLD! 40's during the day, I think evening was 30's. I loved it, but I had to use thermal underwear, gloves, hat, etc.
Day 6, back in the 80's, clear during the day, thunderstorm at night!
So I guess the rule-of-thumb for December is, be prepared for anything and everything!