Basics of the Arizona Monsoon & Desert Meteorology

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!

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. 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 Southwest Area Monsoon Project .

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."

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.

Bursts and Breaks

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" and "Breaks". 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.

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Spotter Training

The first phase 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

Basic storm weather information for Arizona-specific weather is given below:

thunder cloud formation
Schematic representation of a mature thunderstorm

Storm cells in Arizona are generally short-lived. 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.

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.

Haboob: A lens-shaped dust wall generated from surface outflow (see downburst) 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:

microburst diagram

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Downburst. 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.

dustwall outflow
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 microburst
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 microburst
Wet Monsoon Thunderstorm Schematic

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Gustnadoes

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.

gustnado
Figure showing the Guadalupe AZ gustnado (courtesy John Moore)

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.

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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.

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

bufort scale