Free: Contests & Raffles.
Sounds like a heck of a trip. Best wishes for a big tag out I haven't been in the Tetons in September but I was in the Bighorns @ 9,000'+ in October. It was in the low 30s most of the day and occasional winds were tolerable if not annoying. We got lucky I guess with the weather. It had snowed in late September but not at all while we were there. We wore lots of layers and no cotton. Definitely better to have an not need than to need and not have some quality rain gear.
I've been in that country in late August(fishing) and had it snow on us! Gotta plan for the worst and hope for the best. Not so much your daytime clothes as much as your nighttime/camping gear. Not hard to stay dry and warm during the day, but waking up to a foot of snow after a days rain can be murder on your gear if you haven't planned accordingly. Don't know if your being packed in or going in on your own, but even some of the packers don't really prepare properly for a cold onset that early. Beautiful country, wish I was going.