We were last at WDW in December in 1997 but it was pretty chilly and I wasn't really prepared for that. We ended up buying sweatshirts to wear at least one of the days....I looked back to see what the actual temps were and 2 days in a row the low was 46 with the high temp being 64. We wore shorts most of our days so like everyone says, bring everything
This year I plan on bringing a hoodie, a light-weight jacket that I can tie around my waist, jeans, capris, and shorts. I will probably only bring one pair of jeans and will wash them if necessary. I'll have short sleeved shirts and long sleeved. Of course, I will check out the 10 day forecast too. If it seems like Orlando is in a heat wave or conversely a cold spell, I'll adjust accordingly. I'm actually dying to buy a cute jacket I saw at the
disney store (also have it in the parks).

It would be a shame to have to buy some fun disney clothes at the parks because I didn't plan well