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| Southeast Arizona
Monsoon Progression? PDF
version |
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| No two
years are ever alike weather-wise, and the same can certainly be said about
the monsoon. However the monsoon ebbs and flows in a relatively predictable
cycle each year across Arizona, with five broadly-defined phases. These
phases depend heavily on where the subtropical (monsoon) ridge is located,
and how strong it is. |
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| Many
tools are used to track these trends, but the single best method is to
monitor the height of the 500mb atmospheric pressure. This 500mb height,
usually expressed in meters above sea level, indicates how strong the
monsoon ridge is, where it is centered, and how warm it is underneath.
A height above 5950m is indicative of a strong monsoon ridge with hot
temperatures underneath. It also tends to lead to stronger easterly
flow aloft south of the ridge. A height below 5880m is indicative of a
weak monsoon ridge, with slightly cooler temperatures underneath. It
also means weaker easterly winds south of the ridge, and the tendency
for the high to shift south and allow drier westerly winds aloft to
penetrate into the monsoon region. |
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| Ramp-Up:
Mid June through early July |
| By mid
June, the flow of tropical moisture has begun into the Sierra Madres in
southern Chihuahua, Durango and Sinaloa, Mexico. The monsoon ridge has formed,
is strengthening, and is expanding north into the Rio Grande Valley. This
causes upper level winds over southeast Arizona to turn from the southwest
or west to the southeast or east. The shift begins to spread upper level
subtropical moisture from the Gulf of Mexico into northwest Mexico and the
southwest U.S. Isolated thunderstorms develop, mainly during the hottest
part of the day in late afternoon. The lower levels of the atmosphere remain
dry, though, so most of the rain from these mainly mountain thunderstorms
evaporates before reaching the ground. Instead, the storms produce strong,
gusty and highly variable winds, and dry lightning. By this time, the mountain
forests have not received precipitation since April, so both live and dead
vegetation is at its driest. Thus the risk of wildfires is at its highest
as well. |
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| A few
of these weakening thunderstorms may be pushed by the winds aloft into the
lower deserts during the early evening hours. As the rain from these thunderstorms
evaporates into the hot, dry desert air, it cools and accelerates downward
to the surface. This is known as a microburst. Microbursts, while only a
mile or two across, can be accompanied by damaging wind gusts exceeding
70 mph. As these gusts strike the desert and valley floors, a dust storm,
or haboob, can occur. Dust storms like this can reach great heights and
travel dozens of miles away from the thunderstorm. Within the dust storm,
visibilities can drop to near zero. |
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| In this
ramp up phase, the monsoon ridge can shift back to the south, and cause
a temporary return to hot, dry, early summer weather. This "false start"
can be problematic if wildfires have been ignited, since the return to very
low humilities is usually accompanied by stronger winds. Once every few
years, though, the low level moisture does not lag behind the upper level
moisture. Instead, the low level moisture surges into Arizona from the subtropical
rain forests of the southern Sierra Madres, up the Rio Grande Valley, or
up the Gulf of California. If that happens, we can entirely skip this phase
and jump to the Onset Phase. |
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| Mean
500mb heights, June 25 (monsoon ramp up) |
| Click
image for larger version |
|
Visible satellite
image of isolated thunderstorms during monsoon ramp-up, June 28, 2007 |
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| Onset: late June
through mid July |
| As moisture
increases over northern Mexico, and thunderstorms become a daily occurrence
over the Sierra Madres just south of the Arizona border, dramatic changes
take place in the mountain landscape. Areas that were brown and dry in spring
and early summer rapidly green up as semitropical trees begin to soak up
recently-fallen monsoon rains and sprout canopies of leaves. These trees,
shrubs and grasses, combined with the wet soils, put considerable moisture
into the atmosphere through evapotranspiration. Meanwhile, the monsoon ridge
continues to strengthen and move northward into the southern Plains of the
U.S. By now, winds aloft over southeast Arizona are solidly out of the east
or southeast. Moisture spreads into the region not only aloft, but also
at the surface. Surface dewpoints are usually the last weather parameter
to reach critical values as the monsoon arrives in earnest. |
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| At this
stage, the deserts are still relatively hot and dry. However, the thunderstorms
become much more organized, more numerous, and produce more rain. They start
developing earlier in the afternoon, and may continue to grow and organize
into the evening hours as they move west or northwest more easily. Severe
weather becomes an even greater concern as the damaging winds being to affect
larger areas. Dust storms and microbursts may still occur, especially as
the storms spread north and west toward Phoenix and the low deserts. However,
as the first heavy rains begin to fall on the deserts, dust storms become
less common as the risk of flash flooding increases. |
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| Mean
500mb heights, July 10 (monsoon onset) |
| Click
image for larger version |
|
Visible satellite
image from an onset phase severe thunderstorm outbreak over southeast
Arizona, July 14, 2002. |
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| Peak: mid July
through mid August |
| This is
monsoon prime-time in southeast Arizona. The monsoon high is at its strongest
and northernmost extent in the four corners region of the U.S. This high
can meander over the region, and at times may even shift into the Great
Basin. Steering winds aloft are persistently out of the east over southern
Arizona, and may even turn to the northeast. If the monsoon ridge is unusually
weak or is pushed south by an unseasonably strong storm in the Pacific Northwest,
monsoon "breaks" can occur as the upper level winds weaken and
fail to bring additional moisture into the region. |
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| Two particular
phenomena can also take place and cause heavy "burst" periods
of rainfall. First, there are occasional low-level moisture surges from
the Gulf of California, which by now has water temperatures approaching
90° F. These Gulf Surges travel up the Colorado River Valley and low
deserts of Arizona, sometimes even reaching Utah and Nevada. Second, upper
level disturbances rotating around the subtropical high pass rather regularly
across southern Arizona and Sonora. These create a more favorable atmospheric
environment at upper levels for thunderstorm development. When one or both of
these conditions occur, thunderstorms moving westward off the mountain ranges
and into the low deserts may continue to grow and organize, instead of decaying.
The favorable upper level environment can also affect the timing of thunderstorm
development. Instead of the typical development in the afternoon, thunderstorms
can occasionally develop in the middle of the night, early in the morning,
or redevelop repeatedly over the same general area. With the increased moisture,
and more focusing mechanisms in the upper atmosphere, flash flooding becomes
the main concern. Dry microbursts are unusual during this time, but the
rain can be so heavy (sometimes falling at the rate of 4 inches per hour)
that the down-rushing, cold rain and even hail can produce damaging winds
on their own. |
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| Mean
500mb height, August 1 (monsoon peak) |
| Click
image for larger version |
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Visible satellite
image from early morning thunderstorms, 0730am MST July 31, 2006,
during the peak of the 2006 monsoon. Many of these thunderstorms produced
1-2 inches of rain per hour. |
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| Late Monsoon: mid
August through early September |
| It is
unusual for the monsoon to end abruptly. By mid August, the increasingly
lower sun angle in the northern hemisphere becomes less effective in maintaining
the monsoon ridge. The high begins to retreat southeast into the southern
Plains and can even break into pieces as the polar jet stream begins to
shift south from Canada. Upper level winds become more variable, and sometimes
turn to the south or even southwest. Breaks become longer between thunderstorm
events. However, the combination of wet soils and continued rain over Mexico
usually keeps low level moisture in place over Arizona. Because steering
winds become more variable and sometimes light, thunderstorms can still
be slow-moving and pose a serious flash flood threat, particularly in places
where it has been wet all summer. |
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| This is the time of year where the tropical eastern Pacific
becomes a concern. Very warm water temperatures support tropical storms
and hurricanes west of the Mexican coast. Most of these hurricanes move
harmlessly west into the open Pacific. However as the monsoon ridge shifts
south, moisture from these systems can be steered toward Arizona. When this
happens, thunderstorm rains can be enhanced and lead to flash flooding. |
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| Mean
500mb height, August 25 (late monsoon) |
| Click
image for larger version |
|
Visible satellite
image from a late season severe thunderstorm and flash flood event,
August 23, 2005. Note southwest flow aloft and weak trough near the
lower Colorado River. |
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| Decay: late August-late
September |
| By September,
the subtropical ridge has weakened considerably, while the jet stream continues
to strengthen and shift deeper into the U.S. Winds aloft begin to blow more
consistently from the southwest and work their way to the surface. Unless
a tropical system is captured, this southwest flow disrupts both the upper
and lower level moisture feed into Arizona. The soils begin to dry out,
and temperatures begin to cool while remaining relatively warm aloft. The
lack of low level moisture and a more stable atmosphere causes thunderstorm
activity to diminish and the monsoon to fade. |
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| In this
decay phase, though, the weather can still turn active. Moisture lingering
from earlier in the summer remains over Mexico until early October. If any
incoming cold fronts from the Pacific tap into this moisture, thunderstorms
will redevelop. This can be a dangerous time since the cold fronts are usually
accompanied by colder air and stronger winds aloft. This situation, which
is more typical for the Plains states, can lead to rotating, supercell storms.
Although Arizona only sees a few tornadoes a year, they are most likely
to occur during this transition period. |
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| Once every
2 to 3 years, an east Pacific tropical storm or hurricane is steered into
Arizona, either by an incoming Pacific storm system, or by the weakening
subtropical high over Mexico. Most of the time, these systems dissipate
before reaching the border. However, they can induce moisture surges into
Arizona and support larger areas of heavy rain. About once every 10 years,
a system survives long enough to actually cross into Arizona as a minimal
tropical storm. In either case, torrential rains and flooding can result
as the monsoon winds down. |
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| Mean
500mb height, September 15 (monsoon decay) |
| Click
image for larger version |
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Visible satellite
imagery from a decay phase severe thunderstorm and flash flood event,
September 18, 2004. Moisture from the lingering monsoon, and from
Tropical Storm Javier was drawn into Arizona by a strong cold front
moving east across Nevada and California. |
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|
| What
is a monsoon? | North American Monsoon
| Gulf Surges | Monsoon
progression | Monsoon Inter-annual
variability | Severe Thunderstorm and
Flash Flooding patterns | Upper
Level Lows and the Monsoon | Monsoon Safety
| For more reading |
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National Oceanic and Atmospheric Administration
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