I and a friend are planning a 3-5 day hike in the GSMNP in late September or early October. Does anyone have experience with what temps we should expect around this time frame?

I'm asking specifically to figure out what sleeping bag I'll need to bring/buy for the hike. I currently have a 40 degree down bag and I'm hoping that with my bivy, bag liner, and base layer(s) that I'll be able to avoid shelling out the bucks for a warmer bag.

Any advice would be greatly appreciated!