I was there this past December for a few days. Talk about wacky weather. It was in the upper 80's the week before I got there.
When we arrived in Orlando on the 17th it was 34'. We wore our winter coats all day with our jeans. By that night we added hats and gloves to watch the Osborne Lights and Fantasmic.
It was slightly warmer the next day. I was in a light fleece and jeans.
The day we left we were in T's and jeans. We probably could have swam that day but, I never would have went in the pool the other two days.
We are going this December and plan on taking a little of everything. Jeans, sweatshirts, shorts, T's and swimsuits.