I went this past January from 22-29. I'm from Ontario, Canada, so thinking I'll be in Florida, I left my winter coat at the airport in my car. Not so smart! The first two days there I had to layer - I watched Wishes with pajama bottoms under jeans, two pairs of socks, a t-shirt, sweatshirt & hoodie, scarf and gloves (thank goodness I packed those!), and I took a blanket I had ordered for home and thankfully had delivered from Disney shopping. And I still had a red nose! At least the parks weren't busy - I watched Wishes sitting against the fence in the hub with no one in front of me! It was 35 degrees!
By the fourth day, I was in capris and t-shirts and sandals all day, and jeans and a sweatshirt at night.
So it's true - pack for all seasons unless you want to buy jackets at WDW - believe me, they sold a lot of them the week I was there. Good thing I packed a couple of sweatshirts of my own.
I still really enjoyed it, and the lines were pretty much non-existent. Have a great trip!