I agree with everyone else, be prepared for anything! I went in the beginning of December and this is what happened:
Day 1, rained all morning, but temperature was in the 80's, cleared up.
Day 2-5, BEAUTIFULLY sunny, but COLD!!! Temp was in 30's-40's all day. I actually wore thermal underwear, gloves, and a hat!!
Day 6, back to the 80's. We were finally able to go to a water park this day (it was closed on the cold days). Then at night, there was a thunderstorm!!
One thing...if it's a cold day, be sure to layer. i.e. wear a t-shirt under your jacket or sweatshirt. Even though it might be cold outside, Disney more than compensates by making sure the attractions are very warm.