OP, I also live in Michigan. I'm usually the judge whether our horses stay outside or inside during a storm. If its a blizzard, they are always put inside with lots of sawdust and hay to eat to stay warm.
Now if it is a thunderstorm, I kind of decide differently. If the storm has HIGH winds, I leave them out, since I just get scared if the barn could possible come down, the windows break, etc etc. Just my inner safety concern I guess. Regular thunder storms I bring them in.
I think my ultimate deciding factor is on our youngest horse, since once before he spooked at something and ran through his sliding door and got trapped between the door and wall. I just really wouldn't want high winds to make odd noises and spook him. Also, during 4th of july I generally leave them out because of all the fireworks going off, just in case.
Hope this helps! Michigan's weather can get absolutely wretched sometimes!