Scheduled to travel September 15th - worried that it will be too hot - any comments or suggestions
There is a regular forum contributor who lives in the area, so she should be able to help.
I recommend always checking the average weather temp before booking any trip if heat or cold is a concern for you because there is always a different time of year you can choose instead. It is a lovely area which we have been to independently.
Below is a link to Weather Spark, a good site to get average weather (temps, precip, etc. etc.) for any spot around the world for any month or day (based on 20+ years of readings). What is especially useful is it doesn't just give you average daily readings which aren't too helpful if there are big swings over the course of a day like you can experience with temperature, especially in places like the desert, Africa, the US Southwest, etc. Instead you'll see graphs of what to typically expect at any time throughout the day, e.g. at 7:00 am, noon, etc. or when the hottest part of the day occurs. The link is actually for Cape Town, SA, leftover from our last trip- just enter the city and month or specific dates you are interested in on the right side. https://weatherspark.com/d/82961/6/4/Average-Weather-on-June-4-in-Cape-Town-South-Africa
Generally it starts cooling off by mid Sep out here but you'll still have some hot days. May also see rain. Nights cool off considerably because of the low humidity and where the altitude is higher. Be prepared to layer, bring a sun hat and stay hydrated. As someone on the news said this morning, don't wait to drink water when your thirsty. Fortunately it's a casual tour so no need to waste luggage space on fancy clothes. You're going to see some amazing country. Enjoy.
I live in Colorado and have visited the Utah NPs a number of times. I usually go in Oct/Nov. In mid Sept., it is likely to be 90+F in the afternoons at the start of your tour into the Moab area. Bryce and the GC (north rim) should be a bit cooler due to the elevation (the change is roughly 3F per 1000 ft) with potentially chilly mornings. The odds are good for triple digits in Vegas (pun intended).
The scenery in Utah is just magnificent. Too bad you fly across the state, as the roadside views from I-70 could be a national park itself!