It depends how much rain we’re talkin. If it’s just a regular amount, it should have little (if any) effect. If it’s non-stop hard rain or down pouring then it will likely dilute your water and will mess with the FC lost. We had rain about a week ago that I had to drain a few times so my pool wouldn’t overflow and I kept having to go dump bleach in the pool. Super annoying! But a little rain shouldn’t hurt! If debris blows into the pool, it will use up some FC, though. Just a thought.
I ALWAYS add extra liquid chlorine to my pool before rain/storms just to be on the safe side. No algae blooms yet.