I been to Typhoon Lagoon twice in early January - in 2002 and 2003. Those two times were the BEST time I ever had at water parks!

The wave pool has plenty of space so you can body surf without hitting another soul. I went on every tube and body slide so many times that my feet were hurting from climbing all the stairs (at least over 30 times!). I floated in the lazy river without being elbow to elbow with other people. The air temperatures for 2002 was about 77 degrees, and about 73 degrees for 2003. Most people swimming there were of northerners (like me) or from the UK.
Every CM is wearing coats and pants, saying how cold it is! I even saw a young college-age CM sleeping at a vendor stand, since he had nothing to do. On each trip, I was lucky that I had a warm day to do the water parks, as the air temperature for the rest of my trip were in the low 60's or 50's. I never felt too cold there, especially with the sun shining.
Be aware that Disney does close the water parks if they decide the weather forcast says it will be too cold. When I was there in 2002, the CM's already knew the park would be closed the next day, as the temperature was forcast to be 55 degrees.
So I say if you get a sunny day with air temperatures near or above 75ish degrees, then go for it!
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