Weather can change fast up there. It will be anything from snow and thunderstorms to sun beating on your face. Google Historical weather data for the area you will be hunting near to get a better idea. Be prepared for everything. It's an awesome experience.
It is hard the first couple of days up hight but the longer you are there the easier it becomes, just take it slow the first couple days. Hike up high and then camp a little lower and then hike high again and then camp as high as you can w/o exposing yourself to bad weather. Its been my experience that if you sleep higher up your body adjustes faster. starts to make more red blood cells to carry smaller amounts of oxygen.
Invest in a cheap piece of painter plastic. This stuff is lightweight and when that unexpected rainstorm comes, stertch that baby between a couple rocks/trees and have lunch while staying dry. It will likely be cool in the mornings and warm up. But I have also seen snow, sleet and rain that high in late August.
If you use the layering system when you hunt up high you will be fine. Last year durring Sept. I was glassing in a t-shirt and 5 minutes later getting dowsed with rain and a cold wind. Make sure you have a good set of rain gear.