It on avg has the lowest rainfall, but this can be decieving because the showers come and go so quickly. We have been there a total of 14 days in May the last 2 years and have gotten only one afternoon shower that lasted 15 min (Of course we were in the middle of riding Triceratops Spin and got soaked, but that is another story.)
The temps are in the upper 80s and it is not so cool at night that you would need a sweater, unless you are really used to heat. (We were comfortable in shorts and a t in the evenings.)
The crowds are relatively low (with the exception of Memorial Day weekend), but definitely not empty by any means.