Not folk, but I really like Autumn In New York.

I'm also fond of Autumn Leaves, in the original French (Les Feuilles Mort) and in English, as well as September Song, Try To Remember (from The Fantasticks!), and - probably my favorite fall song: Urge For Goin'.

There's another good song about autumn - which I find very difficult to sing because of its range and because there are places you have to have a lot of lung capacity (breath) to get through the lyric phrase without taking an awkward breath.   It's on Dan Fogelberg's album "High Winter Snows," but Jay Bolotin wrote it. It's called "Go Down Easy". (That link will take you to a thread here with the lyrics.)

Other songs that are not really "fall" songs but that I love to sing when the weather gets colder, windier, or rainier include:
Button Up Your Overcoat
Stormy Weather
Baby, It's Cold Outside