Weather wise it could be anything. My daughter was there for two weeks at the beginning of January this year. It was hot out. They swam and wore shorts and tees The Whole entire except one morning it was a bit cool. I was there for a week at the end of January basically a week after my daughter left. It was cold. Wore a light down jacket most of the time with hoodie under brought those hand and feet warmers wore gloves etc.
January is all over the place. I've been there when it's freezing, I've been there when it's in the 80s, and in between. As for crowds, assuming things are closer to normal by then, you're going to have higher crowds around New Year's, Marathon and MLK weekend.