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@ 18-May-2024 10:25pm
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Pepper Ridge North Valley's

Arizona Monsoon Basics Page

What is the Arizona Monsoon?
What is the Mexican Monsoon?
What is a Monsoon Burst ? or a Monsoon Break?
What is a Gulf of California Moisture Surge?
Why do thunderstorms occur during the summer?
Weather patterns associated with severe thunderstorms.
What is a monsoon thunderstorm?
Current Active Lightning Activity

A monsoon ... in ARIZONA? Surely you jest!?

Yes, indeed! Contrary to the denials of Arizona old-timers, we here in the Southwest do have a "monsoon".

How can Arizona, a desert, have a monsoon like India?

Actually one of the reasons that India has its more famous monsoon is largely due to the huge Rajasthan Desert in western India. But more fundamentally a 'Monsoon' is linked more to a wind shift rather than precipitation. In fact, the name "monsoon" is derived from the Arabic word "mausim" which means "season" or "wind-shift". Again, for India, during the winter dry period, the airflow comes from high pressure to the north—the dry Himalayas and Siberia. For the summer, the desert of western India heats up and low pressure forms. This causes air to swirl in from the west, the south and the east—all oceans! The result? HEAVY RAIN!

By the way, the term "monsoons" as in "when the monsoons arrive ..." is a meteorological no-no. There is no such beast. The word should be used in the same manner that "summer" is used. Consequently, the proper terminology is "monsoon thunderstorms" not "monsoons."

The Arizona Monsoon: - is a well-defined meteorological event (technically called a meteorological 'singularity') that occurs during the summer throughout the southwest portion of North America. The Arizona Monsoon, is the northern extension of the Mexican Monsoon. During the winter time, the primary wind flow in Arizona is from the west or northwest—from California and Nevada. As we move into the summer, the winds shift to a southerly or southeasterly direction. Moisture streams northward from the Pacific Ocean and the Gulf of Mexico. This shift produces a radical change in moisture conditions statewide.
Such a change, together with daytime heating, is the key to the Arizona monsoon. This wind shift is the result of two meteorological changes:

  • the movement northward from winter to summer of the huge upper air subtropical high pressure cells, specifically the so-called Bermuda High (H).
  • In addition, the intense heating of the desert creates rising air and surface low pressure (called a thermal low) in the Mohave (L).

These two features combine to create strong southerly flow over Arizona. The southerly winds push moisture north-ward from Mexico. The exact source region for the moisture of the Arizona monsoon is unknown. Researchers have proposed the Gulf of Mexico and/or the Gulf of California as the source regions but conclusive evidence has so far been elusive.This has lead to the creation of large data-collecting efforts and research programs such as SWAMP, the South-West Area Monsoon Project.

What is the Mexican Monsoon?
(The figures and discussion below are based on an article in the Journal of Climate written by Douglas et al. 1993.)

The Mexican Monsoon is a regional-scale circulation that develops over southwest North America 
during the months of July through September. It is associated with a dramatic increase in rainfall that 
occurs over what is normally an arid region of North America.

The term "Mexican Monsoon" is used because of similarities to the better known Southwest Asian 
Monsoon. The similarities between the two monsoons include a shift in the mid-level flow from westerly
to easterly, the mean diurnal low-level flow changes form offshore to onshore, extremely hot and dry conditions preceeding the onset rainfall and a rapid increase in the areal coverage of rainfall during the  early summer.

The Summer Season Pattern Shift.

In June the 500 mb subtropical ridge (18,000 feet above sea level) is located over northwest Mexico. As a result, the flow across Arizona is usually from the southwest. The hot and dry weather conditions  experienced across Arizona during the month of June are a direct result of the position of the 500 mb  subtropical ridge and dry southwest flow.

June mean flow at 18000 feet

By July the 500 mb subtropical ridge normally shifts northward with the center of circulation located  over west Texas and New Mexico. As a result easterly flow develops over northwest Mexico in the mid-levels and hot temperatures over the continent result in a general onshore flow in the low-levels.  The shift in the 500 mb subtropical ridge is followed by a dramatic increase in thunderstorm activity over northwest Mexico. Arizona lies on the northern fringes of this area of enhanced thunderstorm activity. It is during this time that Arizona experiences periodic increases in moisture from the south and east that can lead to thunderstorms.

July mean flow at 18000 feet

Regional Rainfall Characteristic During the Mexican Monsoon.

The map below shows the average rainfall in millimeters over northwest Mexico and the southwest  United States for the Month of July. On average 150 to 300 mm of rainfall occurs over the mountain foothills of northwest Mexico during the month of July (from WMO 1975 Atlas). One hundred and fifty millimeters is about 6 inches, this is a lot of rain in anybody's book for just one month!

Regional rainfall patterns across Mexico and SW United States

Below is an analysis of the contribution of the monthly mean rainfall to the annual total for the months of June and July. Over northwest Mexico only 1 to 5 percent of the annual total rainfall is recorded in June. But look at the July chart, from 20 to 30 percent of the mean annual rainfall occurs in July over northwest Mexico! This gives notice to the dramatic changes that take place in Mexico from late June until early July as the subtropical ridge shifts northward.

Percentage of rainfall that occurs in June across Northern Mexico and SW United States Percentage of rainfall that occurs in July across Northern Mexico and SW United States

The Mexican Monsoon begins in July and lasts through September. The chart below shows in percentage the contribution during these three months to the annual rainfall total. From 60 to 70 percent of the annual rainfall occurs during these three months across northwest Mexico.

This chart shows in percentage the contribution during these three months to the annual rainfall total.

The plots shown below show the frequency of the occurrence of cloud-top temperatures less than -38 degrees Celsius (thunderstorms) for June, July and August are quite revealing. There is a dramatic increase in coverage of thunderstorms between June and July over northwest Mexico and much of Arizona. The images show that the area of highest thunderstorm frequency is over the western slopes of the mountains of northwest Mexico. Arizona is apparently located on the northern fringe of this activity. This explains the variability of the monsoon over Arizona.

June July August
Plot of frequency in the occurrence of cloud-top temperatures less than -38 degrees Celsius in June Plots of frequency in the occurrence of cloud-top temperatures less than -38 degrees Celsius in July Plots of frequency in the occurrence of cloud-top temperatures less than -38 degrees Celsius in August

this is a color bar

Scale: for Cloud top Temperatures

Vertical Wind Profiles and Topographic Influences.

The figure below shows the mean relative humidity and wind speed/direction in the vertical from 
Guaymas, Mexico for each month of the year. Note the dramatic increase in the mean relative humidity
from June until July. At the same time the mean wind profile shifts from westerly to easterly. This is the Mexican Monsoon.

Month to month vertical wind profile

The hatched area in the figure below shows the 3000 foot terrain level. Notice the low level flow 
features on either side of the high terrain of northern Mexico. Note the streamlines moving into the 
central United States from the Gulf of Mexico. Equally interesting are the streamlines from the Gulf of California/Tropical Eastern Pacific into Arizona. This figure illustrates that it is very difficult for low-level moisture from the Gulf of Mexico to make it across the continental divide.

This chart depicts low level wind flow to import moisture north into Arizona

While the subtropical ridge shifts northward over the southwest United States the Gulf of California water temperatures rise dramatically. By early July Gulf of California water temperatures have warmed into the mid to upper 80s (Fahrenheit). With an ample nearby moisture source and steep mountainfoothills thunderstorms develop almost daily across northwest Mexico.

Monsoon thunderstorms are convective in nature. By that, we mean that the thunderstorms are powered by intense surface heating. In addition, strong moisture influx into Arizona is also required. The operational criterion for the onset of "monsoon" conditions used in Arizona is "prolonged (3 consecutive days or more) period of dew points averaging 55°F" or higher." There is nothing magical, however, about 55°F. It originally was linked to the total amount of water in the atmosphere above the weather station (a precipitable water amount of 1", a quantity thought to be necessary for convective thunderstorm activity). In general, for Phoenix, the temperature limits for the production of monsoon thunderstorms are 100° to 108°F with the optimum temperature being about 105°F. Temperatures needed to produce Tucson's thunderstorms are somewhat lower.

The Arizona monsoonal circulation does not produce thunderstorms every day during the months of July-September but rather occurs in a pattern that has "Bursts","Breaks" and "Gulf Surges". According to climatologist Andrew Carleton:

"Burst": a movement into Arizona of a weak trough in the upper level westerlies (normally during summer these winds are far north of this location) which spreads upper level cold air into the region. In lower levels, during a "burst", there is strong surface heating and strong southerly or southeasterly transport of moisture into Arizona. This creates intense atmospheric destabilization and leads to strong widespread thunderstorm outbreaks.

"Break": an enhanced ridging of the upper level Bermuda and North Pacific subtropical high pressure systems which acts to stabilize the atmosphere and thereby cutoff widespread thunderstorm activity. Our own meteorologists suggest that a break usually occurs when the subtropical ridge re-develops over NW Mexico and drier air spreads into Arizona.



The satellite images above illustrates a burst and break cycles that occur over Arizona. During bursts (left) weak areas of disturbance in the atmosphere will cause thunderstorm activity to focus on a small area of the state for a few days to one week. The breaks (right) are when the southerly winds are minimal and atmosphere becomes more stable making thunderstorms unlikely to develop.

Gulf of California Surges
What is A Gulf of California Moisture Surge?

A Gulf of California moisture surge is a low-level flow of moist, relatively cool air that moves northward over the Gulf of California and into Arizona. Surges of moisture from the Gulf of California provide the fuel that feeds the wide spread and intense storms that form over the low desert of Arizona.

What conditions set-up a Gulf surge?

A Gulf Surge can occur whenever the surface based thermal low over the southwest United States is intensifying at the same time large thunderstorm complexes are forming over northwest Mexico. Many of the severe and heavy rain producing thunderstorms that form over the low desert of Arizona during the monsoon are directly related to Gulf of California moisture surges.

The most common large-scale pattern observed before a Gulf Surge is an easterly wave passing over central portions of Mexico at the same time mid-level high pressure and very hot weather are plaguing Arizona. Hot weather normally intensifies the surface based thermal low over the southwest United States.

this shows an  easterly wave

A less frequently observed pattern for a Gulf Surge is when a tropical storm or hurricane is located near the southern tip of Baja California. The realtively cool and moist tropical air mass is essentially pushed northward through the Gulf of California. At the same time Arizona is usually experiencing very hot daytime temperatures with a strong thermal low at the surface.

This shows a tropical system patttern

What does a surge look like?

Of course not all Surges are created equal. Some surges are shallow and weak with virtually no affect on Arizona except to increase the humidity. Surges of moderate depth and strength can penetrate all the way into Mogollon Rim country with a distinct upswing in convective activity across Arizona. Occasionally, maybe once or twice a monsoon season, a strong and deep surge will find it's way into Arizona. This is when the real fireworks begin!

A cross section of a surge

Talk about Thunderstorms!

When a strong and deep surge moves across Arizona the fuel for really big storms moves into place. When a big surge passes through Phoenix or Tucson the temperature drops and the humidity skyrockets. It feels like Houston, Texas for a day! Gulf Surges provide the fuel (moisture) responsible for many of the monster thunderstorm complexes that form and ravage the lower desert of Arizona.

Basic thunderstorm and weather information for Arizona-specific weather is given below:

Schematic representation of a mature thunderstorm

Basic Thunderstorm formation during the Monsoon Season:

Storm cells in Arizona are generally short-lived. They usually consist of either Single cell thunderstorms or multicell clusters of thunderstorms and have lifetimes of about 40-50 minutes.

Single Cell Thunderstorms - are short lived usually last about 20 to 30 minutes, and these single celled thunderstorms are usually not strong enough to produce severe weather. A true single cell storm is actually quite rare. Even with separate appearing storms in weak vertical wind shear, the gust front of one cell often triggers the growth of another cell some distance away. More commonly they are a multicell cluster of thunderstorms.

Multicell Cluster Thunderstorms - consists of a group of cells, moving along as one unit, with each cell in a different phase of the thunderstorm life cycle. As the cluster moves along, each cell takes its turn as the dominant cell in the cluster. New cells tend to form at the upwind (usually western or southwestern) edge of the cluster. Mature cells are usually found at the center of the cluster with dissipating cells at the downwind (usually eastern or northeastern) edge of the cluster. On ocassion these multicell storms can turn into mesoscale convective systems.

Mesoscale Convective Complexes and Multicell Line Storms - These systems form on occassion during the Monsoon. They tend to assoicated with Easterly Waves or when a tropical system has enter the state and enhances the Monsoon.

Mesoscale Convective Complex (MCC) - Is a large organized convective weather system comprised of a number of large individual thunderstorms, that have becomed organized. It normally persists for several hours and may be rounded or linear in shape. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

The multicell line storm (or "squall line," as it is more commonly called) consists of a long line of storms with a continuous, well-developed gust front at the leading edge of the line. These form on ocassion when easterlywaves or tropical systems enhance the Monsoon. The line of storms can be solid, or there can be gaps and breaks in the line. As the gust front moves forward, the cold outflow forces warrn unstable air into the updraft usually at the leading (eastern) edge of the storm, with the heaviest rain and largest hail just behind (to the west of) the updraft. Lighter rain, associated with older cells, often covers a large area behind the active leading edge of the squall line.

Squall lines can produce hail up to about golf ball size, heavy rainfall and weak tornadoes, but they are best known as prolific downburst producers. Occasionally, an extremely strong downburst will accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo. Bow echoes are easily detected on radar but are difficult (or impossible) to observe visually.

There are three basic stages of thunderstorm development: the updraft cumulus stage, the mature stage and the dissipating stage. The lifecycle of a thunderstorm cell going through these stages is, on average, about 30-40 minutes.

The first stage of thunderstorm development is the updraft (cumulus) stage. In this stage, the primary activity within the cell is pronounced vertical uplift. Warm moist air is lifted adiabatically and condenses to form cumulus-type cloud formations. As the updraft stage continues, the formation of towering cumulus begins. Little or no precipitation occurs during this stage.

Towering Cumulus Stage

The second stage of thunderstorm development is the mature stage that is characterized by both updrafts and downdrafts. Downdrafts are associated with air that is pulled downward by precipitation. Normally downdrafts will be found near the leading edge of the thunderstorm cell. The air descending from the thunderstorm will often hit the ground and be forced out ahead of the cell creating a gust front. In the Arizona desert region, these gust fronts will pick up large quantities of dust and/or sand creating a dust wall. The common desert term for such a phenomenon is haboob.

Mature Thunderstorm Stage

The final stage of thunderstorm lifecycle is the dissipating stage. This usually occurs when a large amount of precipitation has been produced and the storm becomes dominated by downdrafts and weak updrafts in the upper regions of the thunderstorm. At the groundlevel, the gust front moves out over a long distance from the storm and cuts off the storm's inflow. This begins the dissipating stage of the thunderstorm. Even though this thunderstorm has dissipated, its gust front may trigger new thunderstorms as it lifts warm, moist, unstable air. Eventually, when storm has completely weaken it begins to lose it shape often leaving only the top anvil remnants of the storm and debris clouds

Dissipation Thunderstorm Stage

The terms below are usually associated with our Summer Thunderstorms

Haboob: - A lens-shaped dust wall generated from surface outflow (see downbust) from a mature thunderstorm cell. The name comes from the Arabic word habb, meaning "wind." Haboobs are most frequent in SW North America during the month os May through September, with most frequent occurrence in June, but they can occur in every month. Their average duration is less than three hours. The average maximum wind velocity is over 30 mph and dust may raise to heights exceeding 3000 feet. This nice image of a haboob on 30 July 1995 was captured by AZTC member John Moore during one of our missions:

Downbust - Localized pockets of intense downdrafts can create severe weather conditions called "downbursts" . A "downburst" is a strong downdraft that induces an outward burst of damaging winds on or near the surface. Downbursts can be large, called a "macroburst" (2.5 miles or large outflow diameter and damaging winds lasting 5 to 20 minutes) or small, called a "microburst" (less than 2.5 miles outflow diameter with peak winds lasting only 2 to 5 minutes). Therefore, "macrobursts" and "microbursts" are severe conditions of downdrafts.

Cross section of a conceptual vortex ring model of a microburst (Caracena, 1982; 1987). (From: Microbursts: A Handbook for Visual Identification by Caracena, et al., 1989)

All downbursts are characterized by a circulation termed a "vortex ring", a vertically rotating circle of air. Downdrafts can be dry or wet. A dry "downburst" is more common during the climatologically drier times of the Arizona monsoon (June & early July), while a wet "downburst" prevails during the wetter times of the monsoon, statistically in late July through September.

Dry downbursts will not necessarily show a solid perturbation from the base of the cloud to the characteristic curl. Instead, a dry downburst is generally only visible when the vertically descending winds hit the ground and pick up substantial quantities of dust. These types of downbursts are common in Arizona and will be particularly evident during the early portion of the monsoon season when there is still little precipitation associated with thunderstorms.

Dry Monsoon Thunderstorm Schematic

Wet downbursts, on the other hand, have the characteristic precipitation curl tracing out the vortex-ring circulation that surrounds the concentrated downdraft within the rain shaft. Most wet downbursts will describe a "foot shape" as the strong vertical winds carrying precipitation hit the ground and curl upwards.

Wet Monsoon

Wet Monsoon Thunderstorm Schematic

Gustnadoes and Dust Devils

Gustnadoes - Are features that seem to combine some of the characteristics of dust devils) and tornadoes. In essence, a gustnado is a tornado-like vortex that appears to develop on the ground and extend several hundred feet upward. These vortices generally develop along the leading edge of an outflow boundary from a thunderstorm cell. Although generally of limited duration, the winds of gustnadoes can be strong enough to cause damage. Gustnadoes are often mis-identified as fires.

For example, associated with the photograph below, team members identified a gustnado occurring along an outflow boundary near the town of Guadalupe. Upon arrival in the Guadalupe area, no evidence of the feature was seen but follow-up discussion with the Guadalupe Fire Department found that the Fire Department had been called out in response to a report of "a downed airplane that caused a huge fire just south of town". They had been unable to find any fire (or downed plane) and were relieved when we informed them that the feature had been a gustnado. Additonal Photographic imagery taken by AZTC chaser John Moore of the 12 July 1995 gustnado can be found at: AZ Gustnado 1995

Guadalupe Az Gustnado in 1998
Figure showing the Guadalupe AZ gustnado cirra 1998

Dust Devils. A dust devil is a vortex of dust-filled air created by extreme surface heating. Diameters range from 10 feet to greater than 100feet; their average height is between 500 and 1000 feet but can extend to several thousand feet. They display some characteristics similar to tornadoes: both cyclonic and anticyclonic dust devils have been observed and large dust devils have been observed with accompanying "suction vortices" (smaller dust devils rotating around the main vortex).


Tornadoes - Luckily, severe tornadoes are fairly rare in Arizona. Although we have many of the weather features (such as abundant moisture, superadiabatic heating, etc.) needed to create thunderstorms of sufficient severity to produce tornadoes, only rarely do we have them all at the same time. In particular, we often during the summer time fail to have a strong jet stream (a narrow corridor of very strong winds generally found about 6 miles up in the atmosphere) overhead. A jet stream often acts as a super vacuum (creating convergence at the surface) as it aids in sucking up the air from the ground.

However, tornadoes have occurred in Arizona and will occur in the future. In particular, during severe thunderstorms (particularly in the cold-front produced thunderstorms of the fall) we will see what is often termed by the media cold air funnels. A cold air funnel is quite simply a funnel cloud, vortex of spinning air. If a cold air funnel cloud extends down to the ground, it becomes a tornado. In other words, a cold air funnel is a potential tornado and must be treated as such. A funnel cloud should not be taken lightly as any funnel cloud has the potential to become a tornado.

Tornado Safety

(1) DO NOT ATTEMPT TO OUTRUN IT. Tornadoes may not move at all... or .... they can, on occasion, move incredibly fast .... 50 mph or more (this is most likely during the fall and winter storms in Arizona). Don't risk outrunning it!

(2) If caught out in the open, proceed to the lowest place (e.g., a ditch, or culvert or arroyo) and drop flat to the ground. Of course, watch for flash flooding!

(3) If caught out in your vehicle, ABANDON your vehicle and find the lowest place (ditch, culvert or arroyo).

(4) If in a building, head to the lowest floor, the center of the building and in the smallest room .. putting as many walls between you and the storm as possible. A bathroom is generally considered a good safety area given the number of pipes in the walls of the building.


Winds - Although tornadoes are rare in Arizona, strong winds resulting from downbursts are quite common during the summer thunderstorm season in the desert. Desert storm chasers should be prepared to estimate distant winds. You should realize there is a natural tendency to overestimate wind speeds from observations.

Guide Estimating Wind Speeds
(adapted from Beaufort Wind Scale for Land Wind Observations, Smithsonian Meteorological Tables)

Storm Spotter Training - The first phase of training is a series of spotter training sessions presented by National Weather Service personnel and members of the Office of Climatology at Arizona State University. Spotter training involves: (a) visual recognition of environmental signatures commonly associated with on-going and/or developing severe thunderstorms (Caracena et al. 1989) and (b) identification of the specific kinds of meteorological criteria necessary to properly assess severe thunderstorm potential; e.g., wind gusts, hail size, or damage patterns (National Weather Service Operations Manual C40, 1990). Specific training is given in observing and evaluating visually unique or distinct atmospheric phenomena in the Southwest United States such as dust walls, dust devils, and eddies or vortices associated with down bursts (Caracena et al. 1989; Fujita 1985). Much of this training complements existing severe storm spotter programs such as the Arizona Skywarn Amateur Radio Network. The spotter training uses the materials and follows the type of instruction suggested by the National Weather Service (Moller 1978). An excellent on-line stormspotters guide can be found at the following URL: NSSL Stormspotter Guide

A good glossary for weather and storm-chasing terminology is available at: Pepper Ridge North Valley's Weather Glossary

Additional monsoon information can be found on the Monsoon Page.

Resources: NWS Tucson Arizona, University of Arizona College of Atmospheric Sciences and Arizona State University's Geographic Sciences Dept

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 Pepper Ridge North Valley Random Weather Facts

Dew point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur. The dew point is an important measurement used to predict the formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high vapor content; low dew point indicates low vapor content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can even use dew point to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight.

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