@OP

My wife and I went through Yellowstone and the Tetons last year in July. Overnight the YNP temps were around 45 and the daytime temps were usually above 80.

This trip topic came up earlier in this year or late last year and my comment was anyone visiting the YNP [on a bike] should check first to see what roads are affected by construction. There were sections through the park that were soft gravel road bed with alternating traffic around crews. Not bike friendly but not a deal breaker.

Here is a link to historical temps in the area September last year. You can tinker with the data as you require..mid September appears to be the tipping point for weather change.

https://www.timeanddate.com/weather/...th=9&year=2018

https://www.yellowstonepark.com/park/national-park-maps

Happy trails..