Carl Brehmer of Principia Scientific International has given the scientific explanation for the intense heat in the South western US. There is not enough water vapour in the air to keep the temperature down. While the temperature in Little Rock Arkansas was 10 F cooler than its previous record high. This contrasts with arid Las Vegas. In the graphic the red line is the temperature profile above Las Vegas at 700m, 6km and 12km in altitude. The blue line is the temperature profile above Little Rock at the same altitudes measured by daytime weather balloons over four days. The "mixing ratio"(g/kg of water vapour in the air) was three times greater in Little Rock so it is clear from the graphic that the extra humidity decreased the temperature lapse rate(the rate at which the air warms as one descends in altitude) in the lower atmosphere above Little Rock down to 6.3 C/km while the temperature lapse rate in the lower atmosphere above Las Vegas was 7.9C/km over the same period. Above 6km the air temperature and lapse rates above both locations are virtually identical as the air cools above that altitude at a rate of 8C/km.
From the graphic one can see that the air temperature at 6km above both Little Rock and Las Vegas is nearly identical at -6C. So as this -6C air descends into the Nevada climate it warms at a rate of 7.9C/km, while at the same time the air from the same altitude at the same temperature only warms at a rate of 6.3C/km as it descends into Little Rock. A nearly 9C temperature differential has then emerged by the time the air from 6km in altitude above each location has descended to 700m in altitude. As the air descends in both locations the work done on the air mass appears as heat. In the more humid climate a portion of that work energy is diverted to keeping the water vapourized ie it turns into "latent heat" and latent heat does not raise the air temperatur