I visited Lamar Valley in Yellowstone National Park last year, and if I had to describe how that time was, the term wet would come to mind pretty quickly. the weather was not perfect in the "I'm on vacation and want nice, sunny weather", but for photography, bad weather is good weather. Lamar Valley was a great location, full of fall colors, mountains, a river, and large mammals. You can sit there all day and see Elk, Bears, Wolves, Coyotes, Antelope, and while I didn't see any, I assume Bison go there as well. Animals aside, you can also get great landscapes of the valley, aspens, and hopefully dark storm clouds rolling in.