Meteorologists and others in the know love to confound newbies to the American Southwest by telling them that a monsoon is not a storm. Rather, monsoon is a seasonal shift in wind. This shift does tend to bring some mighty violent weather with it, however.
Serious monsoons occur in India and in much of Mexico. Acapulco averages 51.8 inches of rain during its summer monsoon and just 3.3 inches the rest of the year. Arizona and New Mexico are on the fringe of the Mexican monsoon. Much of the year, winds there blow from the west or northwest. During monsoon, they bring moisture up from the southwest, from the Gulf of Mexico and the tropical Pacific.
Daytime heating causes the moist air to rise, where it condenses and can form violent thunderstorms. (A wind gust of 115 mph was recorded at the Deer Valley airport in North Phoenix on August 14, 1996.) Still, Phoenix typically gets just 2 to 3 inches from the entire monsoon. The average start date is July 7 and the average end date is September 13. The monsoon is considered to have started when there are three consecutive days when the dew point averages 55 degrees or higher, according to the National Weather Service.
During the monsoon, humidity rises and temperatures typically drop into the 90s and low 100s after peaks above 115 prior to the onset of monsoon.
A storm during the monsoon can bring severe flooding and deadly flash floods. Such a storm can dump an inch of rain in one Phoenix neighborhood and leave others dry.