I have hunted NW Wyoming in October several times.
What I found is occasional rain, occasional snow, cool to cold nights, but it was dry most of the time.
I would NOT spend a lot of money on rain gear. I'd spend it on wool pants and maybe a goretex outer jacket, but that's about it. If it gets sloppy wet, I'd be really surprised.
It is NOT wet compared to the NW (Oregon, WA). The days will still see 50 or 60 degrees and nights will put down some frost. If a front moves in, it may rain a bit and / or snow, but not for extended periods unless it is a fluke storm and it will melt within a day or two.
Bottom line is that even if the chance of rain is low, if it does rain and you are not prepared, you will be miserable. I would not recommend solid vinyl rainsuits that don't breath. You would be sweating profusely immediately when walking in the mountains. Some people get by with a cheap poncho, but I have invested in some breathable raingear. You can spend an arm and a leg, but I got Cabelas packable rain suede jacket and pants for around $150 total. They are nonisulated and are roomy enough that you can put them on over other clothes/jackets. I probably only use them 1 out of 3 trips to the mountains, but they are worth their weight in gold when I need them.
The other item that you need to know is that your boots are more important than your raingear. They should be broken in well and I usually take 2 pair, one insulated and 1 noninsulated. If I had to take only 1 pair, it would probably be the unisulated one and use a good liner and thick wool sock. They definately should be a breathable/waterproof. Cold/wet feet will also make you miserable.
As the allure of hunting big whitetails becomes more and more a passion for many, we are finding that the recent (historically speaking) popularity of hunting deer from a tree stand is becoming the way to do it. I'll make no statement either for or against that technique here. This will be simply an essay on what I feel are some outstanding rifles for tree stand use.
Before we go even one more step, let's all remember that safety is first and foremost for anyone wanting to hunt...