Very unpredictable! We have been during early December three times!
The first time, it was pretty chilly most of our visit, but we did get a few nice warm days. It neve rgot quite warm enough for us to swim, though.
The second time, it was fabulous! Highs in the 80s and lows in the 60s. We enjoyed the pools, waterparks, etc. It was perfect!
The last time, we literally FROZE!! It never got about 50 the whole four days we were there! We had to go out and buy hats, scarves, gloves, etc. On the night we did the Christmas party, it was in the low 30s!! They were putting salt around the exit at Splash Mountain where the water from the logs was freezing and causing people to slip. It was pretty miserable.
However, the low crowds, Chirstmas decorations, etc. make it worth the gamble to us, so we will try this time of year again in the future.