Thunderstorms, or any storms, can be a very traumatic time for your pet if you're not around.
Storms are a part of life, but not for these little guys. We know what's happening...they don't.
The most important thing to do is basically not do anything. When the lightning strikes...stay calm. When the thunder rages, don't move...act like it's an everyday activity.
My oldest was used to them but the youngest peed everywhere whenever a storm hit. So when a storm came around and he looked to me for direction, I did nothing. I acted as if it didn't matter. After the first two times, he lifted his head to watch my reaction and, when there was none, went calmly went back to sleep.
Pets mimic our behavior. If we freak out, they freak out. If we are calm, they are calm. After all, it's just another noice that they add to their repertoire of things to "worry about" and things that "don't matter".