It rains often year round in Zi-tah, but it does it the most in those months.... as part of Noble Mold
's ridiculous spawn pattern (it's probably the hardest NM in the game to realistically camp... harder than Taisaijin, but not as bad as Voluptuous Vilma or Defoliate Leshy). I honestly don't know if anyone's ever documented weather variances in any other months there or at all in any other zone. Even during the rainy season, it's sometimes actually thunder weather with on-screen rain rather than actual water weather.
The whole purpose of my earlier post was to point out that it might
help, but there's no way to know if weather patterns can be predicted for a particular month in a particular zone for sure without actually watching the weather for a zone constantly over the period of several in-game years... and each year is a little over 2 weeks IRL.
Basically, if you're planning on doing several trials in the future that might require the same weather, try to document when you saw that weather and see if there's a pattern you can use in the future... I'm sorry I couldn't offer better advice than that.