 The monsoon begins in late May to early June in southern Mexico and spreads along the western slopes of the Sierra Madre Occidental, reaching Arizona and New Mexico in early July.It extends into the southwest United States as it matures in mid-July, when an area of high pressure, called the monsoon or subtropical ridge, develops in the upper atmosphere over the Four Corners region, creating wind flow aloft from the east or southeast.At the same time, a thermal low (a trough of low pressure from intense surface heating) develops over the Mexican Plateau and the Desert Southwest of the United States.