For the weather watchers out there, Sierra Waves Forecaster Dennis Mattinson reports that the monsoon has arrived. While precipitation that marked a wet month of June came off the Pacific Ocean, like during the winter, the thunderstorms of past days have come from the south and east, a more typical pattern of what forecasters call our summer monsoon.

Mattinson writes, the word “monsoon” is derived from the Arabic word “mausim” which means season. So, what exactly is a monsoon? In this case we are talking about the North American Monsoon, also known as the Arizona Monsoon, the Southwest United States Monsoon, or the Mexican Monsoon.

It is a major shift in the prevailing winds from Northwesterly in the winter to southeasterly in the summer, typically mid-June through mid-August. This wind change is a result of two factors; the movement northward from winter to summer of the large upper level subtropical high pressure system, known as the Bermuda High , and the intense heating of the Mojave Desert creating rising air and surface low pressure, called a thermal low.

These two features both combine to create a strong southeasterly flow over Arizona. The southerly low level winds help to bring in moisture from Mexico (originally from the Gulf of Mexico, the Gulf of California and the Pacific Ocean). When all this moisture meets the higher terrain of Arizona, it lifts and forms thunderstorms. As most of us know, these thunderstorms contain, at times, heavy rainfall, hail, and strong gusty winds. This is then transported southwestwardly and eventually arrives here in our beautiful Eastern Sierra. The storminess also brings lightning at times, which spark mountain fires.