The National Weather Service made a decision in 2008 to give official dates to the Arizona Monsoon season. The start date is June 15th and the end date of the AZ Monsoon is September 30th. It has been helpful to rid us locals of the guesswork so we can concentrate on more important issues like safety precautions.
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 to precipitation.
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. This allows moisture from the Gulf of California and the Gulf of Mexico to stream into Arizona. This shift in the winds, or monsoonal circulation, produces a radical change in moisture conditions statewide.
During a visit to the Mexican Border we happened to be on a mountain and witnessed the formation of a Monsoon storm that would later flow into Tucson and Phoenix, AZ.
Cumulonimbus clouds belong to the thunderstorm clouds or clouds with vertical growth group. Cumulonimbus clouds, also called Storm Clouds, cause heavy rain, lightning, hail, snow and tornadoes.
It became dangerous to be on the mountains so we started our drive home towards Tucson.
This monsoonal circulation is typically referred to, here in Arizona, as the Arizona monsoon. What we experience during the summer months, however, is only a small part of a much larger circulation that encompasses not only Arizona, but 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.
WHAT CAUSES THIS WIND SHIFT, OR MONSOON STORM?
This change in wind direction is the result of two meteorological changes:
- the movement northward from winter to summer of the huge upper level subtropical high pressure system, specifically known as the Bermuda High, and
- the intense heating of the Mohave Desert creates rising air and surface low pressure, called a thermal low
These two features combine to create a strong southerly flow over Arizona. The southerly low-level winds help to bring in moisture from Mexico. When this moisture encounters the higher terrain of Arizona (mountain ranges play a large role), it gets lifted and forms thunderstorms. These thunderstorms can contain very heavy rainfall, hail, strong gusty winds, or a combination of these conditions.
Scientists have a hard time trying to predict storm intensity for the North American Monsoon from season to season because all the small scale processes that drive the Monsoon storms are difficult to catch. The smaller driving forces of the monsoon climate include ocean and atmosphere interactions, land elevation, soil moisture and vegetation, along with the interconnections of land, air and sea. These factors all play a part in what drives the Monsoon Season but it is also why predictability is so difficult.
When such high volume rain descends upon the Valley of the Sun (Phoenix, AZ) the ground and most especially the surface streets flood. Quite often the rain pools on streets during and for a few hours after monsoon storms causing dangerous driving conditions.
The monsoon circulation does not produce thunderstorms every day during the months of July-September, but rather occurs in a pattern that has what are known as “bursts” and “breaks”. During the “bursts”, the Bermuda high will become a bit stronger and develop over northwestern Mexico. This leads to “breaks” in the monsoon, where the southerly winds decrease and the atmosphere becomes much less likely to allow thunderstorms to develop.
This cycle of “bursts” and “breaks” will continue from the onset of the monsoon circulation (typically becoming established near the end of June or the beginning of July), until the time when cold fronts begin to move across the state of Arizona (typically during the month of September), which returns our winds to a westerly or northwesterly direction.
We had a current Monsoon Storm today, September 6th, 2012. My goal was to make Monsoon Season easy to understand. I hope you enjoyed this article.