So I purchased the Seasons EP yesterday, started new game, set starting season to Fall (when asked), went to Options and set season length to 4 weeks (so I would have time to explore all that each sea...
The weather generator in the game probably has a max and min percentage setting for each acceptable type of weather during a particular season. It sounds like the percentage span might need some adjustment, but at the same time, it doesn't necessarily cause the exact same persisting weather for all players. In some of my saves it's raining a lot while in others, almost not. Even during the same season.