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...
You can temporarily turn off rain in the game settings if it's gets too annoying. I know some worlds get more rain than others so you could also try moving your sims to a different world and see if it rains less. I've played in both San Myshuno and Magnolia Promenade and it hasn't rained much at all.