It usually rains each afternoon in summer, so arrive early to get as much done as possible before the rain and/or lightning begin. Have fun! Disney's water parks are great.
The problem is that afternoon rain could hit at any time after noon. It could hit at 12:30, at 2, or at 5. And the rain sometimes goes on well into the evening. So it's safest to tour in the morning before the rain starts.
This may or may not be true. Summer is t-storm season, yes. And they often come in the afternoon. That's also true. But I definitely would not say it "usually" rains each afternoon. It's very hit or miss. We've been to WDW during summer 8 or 9 times, including the last 3 straight...each very long trips. We've had trips where it rained a lot of afternoons. We also had trips, including last summer, where out of the 17 days we were there, we had afternoon t-storms 2 or 3 times only. The rest were sunny days, including a streak of 10 or 11 straight (rained overnight a few times). Storms may not happen at all, they may happen overnight, in the morning, you never know. So don't necessarily plan on it storming in the afternoon. It may or may not.
Water parks are the only place we ever "rope drop". They open at 10am and we show up at 9:55am...plenty of time. Go right to a chair, drop our stuff and hit the big slides. You can go on them over and over and over...the only limitation is how fast you can climb the stairs again. That will last for an hour or so. Then the lines start to build. That's when we hit the smaller slides, wave pool and lazy river. By 2pm, we're ready to head out. You can also do the reverse...show up at 4pm, do lazy river, etc...and do slides late.
Weekends are a bit busier because of locals, but otherwise no real difference in days.
I would not be surprised at all to see FP+ in the water parks this summer, so that will alter things some. They were testing it last summer at BB, and we got to try it out a few different days.