Why does the desert southwest and parts of the Rockies develop a summer thunderstorm season?
The reason is called 'the summer monsoon pattern'. Contrary to popular belief, monsoons occur beyond the tropics. As the Bermuda High settles off the southeastern US over the Atlantic and a low over southeastern California, the circulation around those pressure patterns direct warm, moist ocean air first over northern Mexico, then over New Mexico-typically in May. The combination of sun-heated ground and moist air just above leads to very unstable conditions conducive to producing thunderstorms. As the summer arrives, the monsoon leads to increasing thunderstorm activity (typically during the afternoon hours) over Arizona, SE California, Colorado and Utah.