I didn't see this mentioned, so I'll toss it out there. Apologies if it's off topic.
At least on the Western part of your route, the weather can vary a lot over the same day.
In September, don't be surprised up in the mountains to see morning temps of 25F/-4C and possibly a snow squall or two if a weather front moves through. Later in the same afternoon the temps riding down on the plains or in the desert could still be getting up to 95F/35C (maybe higher across parts of Arizona and Nevada).
In the mountains (out West anyway), rain showers seem to pop up out of nowhere after 2 p.m. and the there are some days when the wind on the plains can be pretty wicked.
It's good to have some pretty flexible gear you can mix and match to handle the changes.