We have been to WDW for most of the past 10 Decembers and it has been hot, warm, cold, sunny, cloudy, rainy, frosty and one time freezing rain.
To paraphrase Forrest, Florida weather in December is like a box of chocolates, you never know what you are going to get.
Just pack a little bit of everything, and you can always layer. Since we usually fly from a cold weather city we have a heavy jacket and then pack a light jacket and sweatshirt to cover all the possible temps. A couple of Dollar store throw away ponchos and we are set.
Dan