Arizona Monsoon brings Microbursts – Describe a Microburst

microburst starts with a typical thunderstorm.  What is a thunderstorm?

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Watching this microburst from our driveway in Southeast Arizona

A thunderstorm works like an engine. It pulls moisture and air in and converts it to rain and then pushes wind and rain out. For the thunderstorm to continue;  it has to be TILTED. The top of the thunderstorm can NOT be directly over the bottom.
During the later months of Arizona Monsoon (which means a season – like summer is a season),  the steering flow in the upper-level of the atmosphere weakens. The UPPER level winds are what tilt storms, such as thunderstorms. The thunderstorm can still form but it will lose the tilt quickly!

This picture is the basics of a thunderstorm. The updrafts and downdrafts are made up of warm air and cooler air.

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courtesy of WVVA TV

(Photo of a Thunderstorm chart courtesy of WVVA TV in Virginia – http://www.wvva.com/ )

As warm, humid air rises inside a storm, heavy rain forms and some of it evaporates in the colder air on top. This cooled air then sinks, accelerates and spreads out as it hits the ground, resulting in a localized, wind called a microburst.

These down bursts are put in two categories. A MACRO-burst and MICRO-burst, only difference is the area they are concentrated in.

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Picture of the Microburst note: the Cumulonimbus clouds were moving towards the microburst and combining with it!

To understand the difference in the sizes of a Micro-burst and a Macro-burst I included another photo from WVVA TV.

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Macroburst courtesy of WVVA TV

A Microburst Storm is an intense, localized downdraft of air that spreads on the ground causing rapid changes in wind direction and speed.   “downburst”

Microbursts are made of winds rushing down to the ground!  Wind speeds can be 50- 100 mph, damaging roofs, snapping trees, etc…

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A Microburst turns into a Macroburst, during Monsoon Season. (moonsoon)

Microbursts can happen so quickly here in Arizona and this is one reason why so many warnings are placed regarding flash floods. These intense storms are capable of producing winds of more than 100 mph causing significant damage.

On a positive note:  Microbursts replenish the desert with much needed rain.

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Desert Monsoon weather, facts and details

What is MONSOON?  The word monsoon is derived from the Arabic word mausim, which means season. Traders fishing the waters off the Arabian and Indian coasts noted that dry northeast winds in the winter suddenly turn southwest during the summer, and bring heavy rains to Asia.

mexico arizona storm weather
summer Monsoon flow graph

We now know that these Monsoon large wind shifts from dry desert areas to moist tropical areas occur in other parts of the world including Arizona.  Strong yearly variations of temperature over land masses is a primary cause of MONSOON.

Tucson Phoenix Monsoon Season map
Monsoon Information weather chart

The monsoon weather in Arizona is not as intense as Monsoon season in Asia and India mainly because the Mexican Pleateau is not as high or as large as the Tibetan Plateau in Asia.  In Arizona, the monsoon process starts with the hot and dry weather of May and June.

Tucson, Phoenix, AZ Monsoon weather storm clouds
Monsoon dark clouds over Coronado Mountains

Most of Arizona’s humid air comes from the Sea of Cortez and the Gulf of Mexico. Our hot desert sun heats the moist air causing the familiar thunderstorm cumulonimbus clouds.

Monsoon weather Arizona storm clouds
Cumulus clouds, Monsoon thunderstorm

Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means “heap” or “pile” in Latin. These clouds typically form when warm air rises and reaches a level of cool air, where the moisture in the air condenses.

If the top of the cumulus cloud reaches above the altitude where the temperature is at or below the freezing level, then precipitation from the cloud is possible. 

Usually by May or June,  our strong Arizona heat causes temperatures to soar over these desert land areas. The intense heat causes surface air pressure to fall, forming an area of low pressure known as a thermal low.

Eventually, the cooler and much more humid air over the ocean is drawn toward the hot, dry air over land. This moist air moving onto the hot land eventually becomes unstable and develops into thunderstorms.

Once this occurs and rain begins to fall, humidity levels increase over land, which only triggers more thunderstorms.  Now you have the Arizona Monsoon Season!

This cycle will continue until land areas begin to cool in the early fall and the monsoon gradually ends.

Dark Monsoon Storm Clouds Arizona
Thunderstorm cumulonimbus clouds

Until the late 1970s, there was serious debate about whether a monsoon truly existed in North America. However, considerable research, which culminated in the Southwest Arizona Monsoon Project (SWAMP) in 1990 and 1993, established the fact that a bonafide monsoon, characterized by large-scale wind and rainfall shifts in the summer, develops over much of Mexico and the intermountain region of the U.S.

Dark Thunderstorm Clouds Monsoon
Monsoon Clouds , Microburst cloud in desert

Rainfall during the monsoon varies with distinct “burst” periods of heavy rain and “break” periods with little or no rain. Monsoon precipitation accounts for a substantial portion of annual precipitation in northwest Mexico and the Southwest U.S.

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.

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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.

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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.