Be advised the weather in the Pacific Northwest from Late September on can be oh so fickle. The Cascades are notoriously unforgiving in the fall. The weather can turn on a dime and give you nine cents change. Just make sure to have the proper gear meaning good rain gear. Of course you might get lucky and the fall will be fabulous but its gonna get cold in the mountains at night no matter what time of year you visit. Good luck and have fun.