We go Jan/Feb. The weather is variable, but not overwhelmingly hot. We haven't had rain issues, maybe a shower here or there. Just this last January, when we got there on Saturday it was in the low 80s. Sunday night, it went down to upper 30s (I think we pretty much always have a night reach the 30s). It can be hard to be comfortable, but I'd rather deal with that than the heat of summer. Major problem is it may be around 50 in the morning, but by afternoon be 70s. Dressing warm for morning makes you hot during the day. I hate dressing in layers, because I hate carrying around my sweatshirt. But as I said, I'd much rather deal with that than the heat.
Crowds used to be great, but it's getting busier. As they say, I don't think there is really a slow time anymore. It may not be as bad as a major holiday or spring break, but waits were quite long while we were there this year.
It's getting harder for us to plan Jan/Feb with school, but I really can't bring myself to come when it's hot (or during the intense crowds of Thanksgiving, Christmas, and Easter). I think you'll enjoy it much more. You can be in the park all day, and not be overwhelmed because of heat.