Many, many pool owners are concerned that rain drastically affects your pool water chemistry. It doesn't.
Common sense tells us that a pool with an average depth of 48 inches receiving 2 inches of rainfall generally dilutes everything in the pool by around 4%.
So, if your CYA was 50ppm, it's now 48ppm!!
If your FC was 5ppm, it's now 4.8ppm!!
pH can take a more significant hit but it's not gonna' drop or rise precipitously regardless.
The REALLY big exception to this is if you have an inground pool that gets flooded. All bets are off if you have surface water draining into your pool.