If he has normal poop as well as this smeary poop, it’s not true diarrhea. With true diarrhea you see ample loose stools, watery or with mucus. No round poops at all.
I think this is a case of intermittent soft cecotropes, ISC. The cause can be stress from the thunderstorm. In that case, it will right itself if you give him lots of hay and cut back on sweets for a little while. If cutting back on pellets makes him eat more hay, you can cut back on pellets, but don’t take them away entirely.
If the problem continues more than a week despite increased hay consumption, you might need to look for a reason other than the stress from the thunderstorm. There could for example be something in his diet that doesn’t agree with him (there are individual differences when it comes to what food rabbits tolerate).
Here are two good articles about this type of issue:
http://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
http://www.bio.miami.edu/hare/poop.html
Scroll down to the sections of the articles that are relevant for your bunny.