We have had every kind of weather imaginable... One year over NYE, it was
colder at WDW than in Chicago! 28deg F! We take squall jackets, down vests,
heavy sweatshirts, gloves, hats, and believe it or not silk long johns! They're
very thin, and pack well.
We also take tank tops, shorts, swim stuff, and warm weather gear. We've
had both in the same trip! You may want to check accuweather.com -they have
15 day forecasts... they are mostly guesses past 10 days, but they can give
you trends. We've had pretty dry weather over the years in December, but
we usually take some sort of umbrellas, etc just in case...
have fun!