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.