July and August is the time of the American Monsoon. Earlier in the summer, the desert areas from northern Mexico northward into the Rocky Mountain states heat up from day after day of sunny weather.
Temperatures reach well into the 100s at lower elevations, and sometimes into the 110s and 120s in parts of Arizona and California. The hot air becomes less dense, which results in lower barometric pressure.
During July and August, air flows in from all around in response to the lower pressure, but mountain ranges block much of this movement except for a stream of tropical air from the eastern Pacific which comes by way of the Gulf of Baja. The higher humidity in this air leads to frequent thunderstorm activity over the mountains of the southwestern United States. Ironically, this is also the peak of the Southwestern fire season due to the fact that these storms are notorious for producing widespread, intense lightning but only isolated heavy rainfall.