We were at WDW the first week of December last year. My best advice is to pack for and dress in layers.
The mornings would find us wearing jeans and short-sleeved tee shirts with long-sleeved tee shirts over them, and lined windbreakers.
We felt fine (we had come from N.J. where the am temps were in the 30's at that point, so the 50's felt fine to us!). But some of the CMs were wearing heavy winter jackets, hats, and gloves!
By mid morning, we could either remove our windbreakers and just go with the double tee shirts, or keep the windbreakers and remove one of the tee-shirts. By the afternoon we were sometimes down to just the short-sleeves.
We had three afternoons/evenings when it rained steadily; not the short, intense showers of summer but rather a steady, chilly rain. We were OK with our lined, hooded windbreakers and the double tees.
It never felt quite warm enough for shorts and we did not feel it was warm enough to swim; although the pools were heated and we did see many who were having a good time there.
Our night at the
MVMCP we were at the MK late and our hands got cold so I had to buy gloves for everyone; so you may want to pack those as well.
Be prepared for just about anything!
Kathy