We've been doing WDW in early December for 20+ years and our experience has been varied to say the least. Probably the most notable thing is pretty much a lack of rain. Yes, it may rain (and I recall getting rained out of one
MVMCP) but that's rather unusual so don't worry about rain.
But temperatures are an entirely different issue. We have pictures from one year where we were wearing shorts/t-shirts on Friday and heavy winter coats on Monday. The best strategy is to pack layers. Some combination of t-shirt/polo + sweater/sweatshirt + windbreaker/light jacket will be appropriate for whatever you encounter.
About 10 days before you head for WDW, start checking the long range forecast for WDW (
www.weather.com and use 32830 for the location) to get an idea of what is coming. But don't believe that's really going to happen. Use this only for a guide as to the extremes of weather you may encounter while you are there so you can adjust your packing to suit.
And don't worry if you make the wrong choices - you'll have plenty of opportunity to buy appropriate clothing there.