If its going to pour, take a proper rain jacket. You'll also want gloves, hat etc. as if it rains up at 2000m it's also <5deg and you'll freeze on the way down. Just take a small bag.
I went up in the Alps in September a couple of days before it snowed and tied a LS Gabba around my top tube for the way back down along with some gloves.
Definitely worth taking a framebag or something to keep shit out of your jersey pockets as going up can get hot as hell.
Check the weather before you leave in the morning?