You'll probably be grateful for those thunderstorms. They won't last all day, and they do offer a brief respite from the brutal heat. Of course, it can be pretty steamy once the sun comes back out, but when the storms are coming across or are nearby, at least the sun isn't baking you for a little while and there is often a breeze.
I remember being concerned about the 10-day forecast for our last trip. It showed 40-60% chance of rain and storms every day we were there. This was Easter 2016, and my visions of beautiful springtime at Disney with Flower and Garden and lounging by the pool, etc. were ruined with that forecast.
We had ONE afternoon/evening where rain was a disruption, and even that was kind of fun. My mom and I still reminisce about that day as one of our favorites as we took cover in the shops on Main Street USA to wait out a pretty strong thunderstorm that was dumping buckets of rain down. We watched the mass exodus of people fleeing the park, laughing when the teenage girls were screaming when a crash of thunder would scare them. The Dapper Dans had come inside where we were and were entertaining the guests sheltering in the shops. We watched the glass-blowers work on their creations, ogled the Dooney & Burke bags, tried on hats, and when the storm had eased up enough, we donned our ponchos and sloshed off towards Columbia Harbor House to grab a bite. We left MK soon after and headed to Epcot. It was still drizzling/sprinkling, but we had some fastpasses, so we rode our rides and then went and got snacks from France. The skies finally cleared as it was getting dark, and we had a fastpass for IllumiNations, which was spectacular. We hit MouseGears gift shop on the way out of the park that night, loaded up on junk we didn't need but couldn't live without (LOL), and exhausted but fulfilled, went back to the resort.
That was the last day/night of our trip, and it's such a fond memory, rain and all. I realized then that being in Disney in the rain is still better than not being in Disney at all.