See:Io, what's the chance of getting the rainy weather forcast? I've been trying to get it for 16 ingame days straight and I still got nothing!
https://f95zone.to/threads/daily-lives-of-my-countryside-v0-3-0-milda-sento.46548/post-13504607
https://f95zone.to/threads/daily-lives-of-my-countryside-v0-3-0-milda-sento.46548/post-13506504
*** Edit ***
The probability for a rainy day/night for the NEXT day during the weather forecast is also 1 chance out of 3.