I am asked this question quite frequently.  And, quite frequently my answer is, "There will be weather; I can't say if it will be good or bad, but there will be weather."  Indeed, what other job can you be wrong more than 50% of the time and still get paid, other than weather forecasters?

Nevertheless, here are some historical averages:

May----53High 36low (.95 rain)
June----59High 43low (1.09rain)
July-----62High48low (1.75rain)
Aug-----62High46low (2.62rain)
Sept-----55High39low (3.31rain)

These are averages and the weather is just like fishing, no guaranties.  We have experienced freezing in June and 80's in July and August.  You just never really know.  So the moral of the story is "Be Prepared!"