DH and I just got back from our trip, ending Dec. 8. We've also been to WDW in January. We packed for warm weather and cooler weather. We had days where the high was 53, then a few days later the high was 88 with high humidity. We also had an evening with an absolute downpour, and very cold windy days. The weather is so unpredictable! We would look at the weather forecast the night before and again the morning of to determine what to wear for the day. On most days I layered my clothes, wearing a t-shirt, a thin jacket, and then a thicker fleece jacket. Then I could take off a layer as the day got warmer. I carried a backpack, but you could also use the lockers in the parks if you don't want to carry around extra clothing. I also carried an umbrella and a few disposable ponchos in my backpack. On most days I needed both jackets in the early mornings, and again in the evenings.
Have a magical vacation!