Here's some other locations with long term records. All of these have far less UHI than Phoenix, SLC, or Vegas.
Santa Fe: 4 (but missing a ton of years prior to 1997)
Roswell: 5
Las Vegas, NM: 3
Gallup, NM: 4 (records only back to 1973)
Mesa Verde NP: 2
Imperial, CA: 3
Barstow, CA: 5
Blythe, CA: 2
Winslow, AZ: 5
Prescott, AZ: 3
The mean is around 3-4. Clearly Phoenix's 7 is an outlier, as asserted.