I took 10 days last fall and did a solo from Westwater to Hite, winding up on the lake on November 1st. I had cool nites and beautiful days. There was ice in the water bottle the first several mornings, but below Potash, it was in the 40's overnite. Decent water temperatures -- I ran the rapids in shorts and t-shirt.
There was about thirty miles of lake then, and there almost is sure to be more lake this coming fall, so think about bringing a kicker.
On weather, there are other possibilities, of course. Three Octobers ago, I did Sand Wash to Hite and had a full week of cold, rainy weather on the Green. Pulled into the Confluence with slushy ice on the deck of my cat, and was about as miserable as I've been in a long time on a river. A layover at Spanish Bottom was in order, while waiting for some folks coming down from Potash. And lo and behold, the next day it was sunny and warm. Shorts and t-shirts for the rapids again.
Fall water levels are generally low, of course, but can vary a bit with the contemporary weather. Last Fall I had about 8k cfs. The trip three years before, it was 12-15k cfs because of the rain the week before. But at any of those levels, the rapids are pure fun.
On the solo trip I scouted Rapid #15, BD2, and BD3, just to be safe and because I promised my wife I would. But I'm an old geezer, and (even though prudence might dictate scouting) you young bloods probably will just blast right on through fueled on testosterone.....
Have a good one.