Hiking The Smokies in September - Weather to Expect?

I will be hiking The Smokies for about a week starting September 14th. I hiked The Smokies during my 2013 thru hike, but that was in the month of May (cold rain the whole time). I was wondering if anyone could give a sense of what kind of weather to expect in The Smokies in the middle of September? Would a 3 season sleeping bag still work or should I go with my winter sleeping bag? Thanks for the help in advance.