Originally Posted by alzyck
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.
alzyck makes a good point...
I didn't even get into that in my post but here's an experience I had last year: Mid September 2009 I took a ride out to New Mexico planning to go South to North on the Continental Divide Trail. I knew weather might become an issue so I planned to bail out to lower altitudes if it got nasty. Never made it out of Southern Colorado. Spent 2 nights in 14 deg F temps in a 25 deg bag. BBBRRRRR. After the second night I spent an entire day at Pagosa Springs thawing out.
Lots of snow at higher elevations so I bailed on that plan and saw some more New Mexico.
The northern part of your route might be perfect weather in the first week of September. It might also be really awe inspiring crappy. Have flexible plans and don't beat yourself up if you don't hit every place on that map.