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School of Geographical Sciences
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.
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.
return to top of page <#monsoon>
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
return to top of page <#monsoon> *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
return to top of page <#monsoon>
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).
return to top of page <#monsoon>
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.
return to top of page <#monsoon>
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)