If you go in the summer, it is going to rain- as the other nice folks on this board have been saying. I plan on at least a brief storm most every afternoon I have to be in a park. If it doesn't rain a few of the days I am there I consider it a bonus

You will probably have some days free of rain, just check the weather channel in the AM. There are a few times during the summer where it won't rain a for several days in a row (at least that is how it was when I lived there). I always hope my trip will fall during one of those times. I would never go in July or August, as when I lived there these were the two rainiest (and hottest) months of the summer. We go in June (leaving in 9 days- woohoo) when we visit in the summer. If 'rainy season' patterns have changed, hopefully a current resident can set me straight.
I carry a pair of Tevas (athletic shoe type sandals) in my backpack and take my sneakers and socks off when the storm hits and put the sandals on. The little bit of extra weight in the backpack is worth having dry shoes during the trip. If it looks like it is going to rain all day, I wear the Tevas all day that day with a pair of socks. Take a couple of towels for the group, and a smaller one to sweep the rain off seats. We use the towels after Spash and Kali on sunny days, too.
I pack 2 ponchos per person in case they rip. If they aren't needed, I keep them with my 'Disney stuff' for the next trip. Many days of our trip we get started at park opening and plan a swim and a nap in the middle of the afternoon, as that is when it normally rains (plus that is when it is the hottest). Those days I usually don't take the sandals with us, unless it is an AK or MK day- we always put them on before we ride Splash and Kali (as well as the ponchos on Kali)
This is an early post before the coffee kicks in- so hopefully it makes sense and is helpful
Blessings,
mic_key_mouse