When does Monsoon season start? What causes Monsoon?

The official start date of the Arizona Monsoon season is June 15.  In 2008, the National Weather Service determined the AZ Monsoon starting date along with the ending date of September 30.

Tucson Phoenix Monsoon weather
storm clouds Monsoon season

What causes monsoon?

Traders sailing the Indian Ocean and Arabian Sea used the word monsoon to describe a system of alternating winds which blow from the northeast during the northern winter and from the southwest, during the northern summer. Therefore, the term monsoon refers solely to a seasonal wind shift, and not precipitation.

Mexican monsoon cloud
Arizona Mexico Border storm clouds

Arizona happens to be located in the area of the United States that experiences a monsoonal circulation. During the summer months, winds shift from a west or northwest direction to a south or southeasterly direction.

Monsoon weather for Arizona
Microburst clouds during Monsoon

This wind shift allows moisture from the Gulf of California and the Gulf of Mexico to stream into Arizona.  A monsoonal circulation produces a radical change in moisture conditions throughout the entire state.

Arizona thunderstorm dark clouds
Monsoon storm clouds over Coronado Mountains

Thunderstorm or cumulonimbus clouds are very familiar during monsoon season.  These storm clouds cause lightning, hail and heavy rain.

Arizona desert Monsoon Storms
dark thunderstorm burst clouds

What Arizona experiences during the summer months is only a small part of a larger circulation that encompasses much of the southwestern United States and northwestern Mexico. Thus, it sometimes is also known as the Mexican monsoon. The National Weather Service calls it the North American Monsoon.

microburst monsoon storm cloud
Monsoon clouds Mexican Arizona Border

This adjustment in wind direction is the result of two meteorological changes:

  1. the movement northward of the huge upper level subtropical high pressure system, known as the Bermuda High,
  2. and the intense heating of the Mohave Desert creating rising air and surface low pressure, called a thermal low
severe dark thunderstorm cumulus cloud
Monsoon Storm clouds on top of mountains

The southerly low-level winds help to bring in moisture from Mexico. When this moisture encounters the higher terrain of Arizona mountain ranges, it gets lifted and forms thunderstorms.

dark storm Clouds on arizona border
Monsoon storm Coronado Mountains

Small driving factors such as: atmosphere interaction, land elevation, soil moisture and vegetation all play a part in what drives the Arizona Monsoon; but it is also why predicting storm intensity is so difficult.

When such high volume rain descends upon the Arizona desert the ground and the surface streets flood. Quite often the rain pools on streets during monsoon storms causing dangerous driving conditions.

cumulus thunderstorm cloud
Storm burst clouds at Arizona Mexico Border

The monsoon circulation does not produce thunderstorms every day during the summer months, but rather monsoon storms occur in a pattern known as “bursts” and “breaks”.

best Microburst storm cloud photo
Monsoon microburst cumulus cloud

This cycle of bursts and breaks will continue from the onset of the monsoon (typically June), until the time when cold fronts begin to move across the state of Arizona (typically September), which will return our winds to a westerly or northwesterly direction.

We hope you enjoyed this article.  Our goal was to make Monsoon Season easier to understand.