Noctilucent clouds supposedly only occur between 50 and 70 degrees latitude during that hemisphere’s summer months. But that’s more a function of the sun’s angle below the horizon being in a Goldilocks zone (>/= 6 degrees) for a much longer duration, and covering a much broader area. Truth is noctilucent clouds can be observed anywhere on the planet, if even for a fleeting moment. So not that unusual that they were spotted over Arizona. I’ve seen them here in the Florida panhandle as well.