Well I did take a look and it seems, when changing the weather info in onItemRightClick everything is working fine. In onItemUse the same method block does not work.
When creating a an entity in both methods and then trying to change the weather info in the entity class it will not work either.
Edit:
Holy ...... I solved my problem. And as always .... such a simple trick.
I have a WeatherEntity, that is extended by AntiRain and Rain Entity. Turns out my WeahterEntity still had some code to turn on the rain and my AntiRain Entity was calling the super method. So it disabled the rain and then turned it on again. Heck... time for weekend