If this the not a bug but a feature please add a button to the Weather Editor that take this new weather conditions and set it for the play mode. Actual it is not intuitive.
For now it can be changed like:
* duplicate the state file in the weather and state manager
* inside the file set the "start state" to the desired start weather (e.g. "Cloudy")
* save it and point the manger to the this state file
Proposed solution:
* on creation of a new terrain or as a check by opening the Weather Editor Panel it would be the weather state file would be created in the mod project or duplicated if not present and set as the state file in the weather manager
* on hitting on "OK" in the Weather Editor Panel it would be saved to this state file