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...
In the save I opened after installing Seasons, the first weather I saw was rain (the household I opened lives in Newcrest) . I started a legacy style save the next morning though and haven't seen rain once. In that save I've gone through 14 days of winter and 5-6 of spring without rain, even when my sim has visited other areas besides the one she lives in (Windenburg).