Your weather in January can vary so much. Your best bet is to check the forecast right before you leave. In 7 January trips, we had one really cold week ( Jan 03) . The whole east coast was under a cold snap ( it was -10 back home in Pittsburgh and that wasnt counting the wind chill) So it was low 50's for the high, 30's for the low. One morning we woke up on that trip and it was 28. It was a state of emergency for Orlando and the schools were closed. My kids got a kick out of that. Our school dist. was delayed 2 hours for -30 wind chills.. and Orlando was shut down at 28. haha. Now the good thing was.. when we stayed for the 2nd showing of SPectromagic. There were only 3 other families we could see in Frontierland!!!

It was freezing, but we had the attention of every single character in the parade, so it was awesome!
Most of our other trips have been warmer, rather than colder. We have lucked out where the lows were in the 60's and we stayed in shorts the entire day and night. Sometimes its cooler in the morning, but by afternoon its really warm. SO we wore shorts/capris and light jackets, and brought jeans and sweatshirts for the night and stuck them in the locker ( we did this over thanksgiving too... ) WHen the sun went down, we all changed. We have also had very little rain in January. In 7 trips, its rained maybe 4 times total. Our first trip, it was beautiful and sunny, we came out of Star TOurs and it was pouring!!! we were like what?? rain ?? We all bought our yellow mickey ponchos and decided to go eat lunch since it was raining. By the time we finished up with lunch, the skies were blue and the sun was blazing. I dont think it rained on us again until January 04. January 04 it rained the entire day we were at MGM. We even had a rain poncho for the stroller. It rained one day ( starting around 3 or 4) this past January 05 and rained the whole night. It was still warm out, so the kids swam at the BWV in the rain. It did kill our plans for EMH night at MK. The next day it wasnt raining, but it was blustery... cloudy.. windy.. colder.. Other than that, the weather has been warm, or cooler and comfortable. The weather changes so fast in Floriday that time of year, that you really will have to wait until a day or so before you leave to get a better picture of what the weather will be. Chances are you will need something for warmer day weather, and something for cooler evening weather. We always bring our rain ponchos incase, and I always throw in some of those $1 stretch gloves. Coming from Pittsburgh, we have our heavy coats with us, but pack lighter ones. One the one trip ( 03 ) was it cold enough for our winter coats.
Enjoy January at Disney.. we love it!!