What Bob and Crazywig said is exactly correct. It could be shorts and flip flops weather, or it could be bundling up weather. You never know. Impossible to plan.
Crowded? No way. January is one of the least crowded months, if not, THEE least crowded. In fact, January is typically when Disney offers it's reduced ticket prices for FL residents (because it's the least crowded then). January is usually FL Residents Month, but don't worry, it doesn't add much at all to the crowd, unless it's on MLK Jr. holiday weekend.
The best time that my friends/family and I would go to Disney would be right before a major holiday. Like, 7-10 days before Xmas, for example. You walk on every single attraction that Disney offers. Lines don't exist for eating, attractions, toilets, etc. It's great!