We plan for rain daily, and rejoice if it never comes. Clouds are very welcome though on hot days. Not always, but often, you'll get afternoon/evening showers, and since IoA has mostly outdoor attractions that will shut down if lightning is in the area, do IoA as much as possible in the good weather. Bring ponchos and/or rain jackets. Shoes that either can get soaked, or extra pairs of shoes to switch out the next day.
We also bring plastic bags, or something to hold phones/keep them dry. And either a water proof backpack, or just prepare for it to get wet.
It's difficult to do stuff like wand spells, or lots of picture taking in the rain, so try and get those done when it's clear.
Knockturn alley is a good hideout spot.
our forecast currently has 50-60%rain, so we'll be prepping for/expecting it. That forecast will change FREQUENTLY before and even during the day, so keep monitoring.
If they're open, ride the water rides like Ripsaw falls, and Buto. You're already wet, so just get wetter.
have a great time!!!