What does the
Current 500 MB Chart for the East Pacific Mexico & Southwest U.S. map
show me and why is it so important to the state of the North American Monsoon?
The 500 MB chart shows wind flow at about the mid levels of the atmosphere or
about 18,000 ft above sea level. The flow at this level heavily influences
the movement and development of weather here at the surface.
Late Spring: the Dry & Windy Season:
The beginning of the dry season in Arizona (April & May) will usually have a
pattern as represented in the map below. The Jet Stream has retreated far
enough north to keep pacific moisture confined to higher latitudes, but remains
close enough that Arizona experiences the southern end of these low pressure
systems and the associated strong winds. Dry air at the surface and aloft
typically covers much of the eastern Pacific and southwestern U.S.
Meanwhile, across the dryline, moisture ebbs and flows south and north across
the Plains, Midwest and Gulf Coast. This dryline can be the focus for
severe weather on the plains as it moves east and then retreats west. With
strong westerly flow aloft, this moisture almost never makes it across the
continental divide. To the south, the subtropical high pressure ridge at
500 MB has started to develop over southern Mexico with moist tropical
easterlies to the south of the ridge.
| Common 500 MB Pattern in April & May |
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Summer: The beginning of the Monsoon Season:
Map #1 shows the typical 500 MB flow in late May and June. The 500 MB
subtropical high has become established over west central Mexico.
Southwest winds at 500 MB off the cold waters of the Eastern Pacific do not
contain much moisture and what little moisture remains is wrung out over the
Peninsular and Transverse ranges of Baja California and California.
Occasional bouts of high cloud is all that remains. Dry and increasingly
hot weather is the norm as the high slowly heads north, while the windy and
cooler weather of Spring associated with upper lows and troughs retreats to the
Pacific Northwest with the Jet Stream. Tropical moisture is located south of the
high and flows eastward into the tropical Eastern Pacific or is deflected
northward from the Gulf of Mexico and southern U.S. by the westernmost extent of
the Mexican subtropical upper high's Atlantic counterpart, the Bermuda High.
Map #2 shows the typical 500 MB flow in late July and August. The 500 MB
subtropical high is now centered over the southern plains into New Mexico.
The 500 MB flow has become easterly to southeasterly over Arizona, with the dry
westerlies deflected northwest or southwest of Arizona. Mid level moisture
is transported westward from the Gulf of Mexico and Northwestward from the
tropics as low level flow brings moisture from storms to the east and south and
Gulf of California moisture from the southwest, either in a
surge or a small
daily flow. Mid level disturbances on the south side of the subtropical high
called "easterly waves" ,"inverted troughs" or "tropical waves" can bring
periodic enhancement to the thunderstorm coverage.
Map #3 shows one of the most common variations of the Monsoon Season, the
"Classic: or "Type I" pattern. In this particular pattern, the subtropical
500 MB high has relocated to the four courners area. Deep easterly flow
transports plenty of moisture into Arizona. The position of the
upper high is not static during the season. Upper troughs in the westerly
flow over the Pacific Northwest can occasionally be strong enough to shove the
upper high southward into Mexico again for a breif return to dry weather and
southwest flow; such scenarios happen more and more frequently from mid August
on. The upper high can relocate to southern Nevada, placing strong
northeast flow over Arizona and setting up a scenario of thunderstorm
development called a "Rim Shot" over southern Arizona. This pattern
involves strong storms developing on the Mogollon Rim, and depending on
moisture, surging southwest into central and Southern Arizona mid to late
evening or even overnight. The upper high can relocate directly overhead
or very close by with light winds aloft keeping storms more isolated, near
mountains and relying on their outflow for further development. With ample
moisture, this can create a situation involving very slow or stationary storms
which dump flooding rains while other areas nearby remain rainless.
In summary, the subtropical 500 MB high's position plays a critical role in the
monsoon with each variation of the high's position creating a unique setup for
each day's weather.
| Typical 500 MB Pattern in
late May and June |
Typical 500 MB Pattern in late July
and August |
Typical "Type I or Classic"
monsoon 500 MB Pattern |
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What does the Current Water
Vapor Image centered over Arizona show me and why is this image so
important to Monsoon weather?
Water vapor images show moisture in the upper levels of the atmosphere,
generally above 500 MB and are an excellent guide to the location of upper air
features, particularly wind flow and areas of moisture versus dry air. In
this image, the more white or green the more moisture, while the shades of dark
grey to copper show dry air. Areas under the dry air are much less
likely to see thunderstorms, with isolated coverage at best, while areas under
the moist zone will see much greater coverage. Along the boundary between
dry and moist air is a frequent zone of thunderstorm development, termed the
moisture boundary, or sometimes the monsoon boundary or much less frequently,
the dryline. A loop of this image will show the direction of movement of
these air masses and can be a good indicator of future thunderstorm development.
This image is most useful in the Monsoon season and less useful the rest of the
year as subtropical high clouds or passage of upper level troughs and lows will
not necessarily delineate areas of thunderstorm development.
| Sample Water Vapor Image |
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What other situations produce thunderstorms in the Monsoon Season?
There are several scenarios which may produce thunderstorms during the Monsoon
Season or enhance already present thunderstorms.
Map 1: Gulf Surge with an Easterly Wave:
High pressure aloft over the southwestern U.S. can produce a period of very hot
dry weather which intensifies the surface-based Thermal Low (or Heat Low),
causing the hot air to rise and pull in the surrounding air and moisture from
the Gulf of California. This creates a pressure difference between lower
pressure over the interior southwest and northwest Mexico and higher pressure
over the Gulf of California, also causing air and moisture to flow northward.
At the same time an easterly wave (sometimes referred to as an inverted trough
or tropical wave) is approaching from central Mexico. This wave enhances
thunderstorm complex development, which in turn can feed more moisture up the
Gulf of California and into Arizona.
Map 2: Tropical System induced Gulf Surge:
In this situation, high pressure aloft over the southwest U.S. is providing
hot weather with a strong Thermal (Heat) Low at the surface. The hot air
rises, causing the surrounding air to be drawn in to replace it. At
the same time a tropical cyclone passes west-northwestward to the south of the
southern tip of Baja California. The counterclockwise flow around the
tropical cyclone will push moisture northward up the Gulf of California, with
this flow becoming part of the airmass drawn into the Thermal Low.
Map 3: Thunderstorm Outflow: This is a
common situation as the Monsoon becomes established. Thunderstorm
development begins to the east and south over the higher terrain, rim country
and mountains of Mexico. Cool, moist air released by evaporation flows out
from these storms, acting to moisten the air at the lower elevations. This
in turn produces the next generation of storms further south and west, which in
turn, release cool, moist air, fueling storms progressively closer to the low
deserts.
| Gulf Surge with an
Easterly Wave |
Tropical System induced Gulf
Surge |
Thunderstorm Outflow |
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Map 4: Moisture increase from a "back door" cold front:
On occasion, especially early in the summer, strong cold fronts form over Canada
and drop into the the Plains with a strong surface high pushing into the
southern Plains. This type of scenario can push a cold front southward
across the Front Range of the Rockies and cause it to "backdoor" or push
westward (cold fronts usually move eastward) across New Mexico and into Arizona.
By the time these fronts reach Arizona, they are typically very weak, but can
push low level moisture from the Plains westward into Arizona, fueling
thunderstorms.
Map 5: Mid Level Moisture Transport: The arrival
Mid Level moisture is often the first sign the Monsoon Season is starting.
Depending on the flow aloft, this can bring moisture from thunderstorm complexes
on the High Plains or front range westward, or on the Sierra Madre Occidental
Range in Mexico northwestward.
Map 6: What is a "Rim Shot" in terms of the Monsoon Season?
The scenario in which a "Rim Shot" develops in the Monsoon season is
more likely when strong northeast flow exists at mid levels from the Mogollon
Rim of northern and east central Arizona southwestward across southern Arizona.
This can happen when the 500 MB subtropical high locates over Las Vegas.
While light winds prevail around the high, stronger anticyclonic flow exists
further out. Thunderstorms develop on the Rim early to mid afternoon and
aided by strong outflows and northeast flow drop southwest through the
afternoon. In a situation where plenty of moisture at low levels resides
across the low deserts, these storms arrive with gusty winds and blowing dust
across southeast and south central Arizona during the evening or possibly
overnight hours. If enough moisture is present, these storms can make it
to the Colorado River Valley. If moisture is scarce across the low
deserts, the storms will dissipate in the high country, never making it to the
population centers of Phoenix and Tucson, only bringing gusty winds, blowing
dust and an increase in relative humidity.
| Moisture from a Back
Door Cold Front |
Mid Level Moisture
Transport |
"Rim Shot" Thunderstorm
Episode |
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Thunderstorm development begins over Mexico and progresses north
The following images illustrate the northward progression of
thunderstorms as the North American Monsoon becomes established. These
colorized infrared images show the temperature of the cloud tops and of the
surface. The heated ground of the Desert Southwest becomes evident in
the deep blue and purple shading. Lighter blues, greens, yellows, oranges
and reds show the progressively colder (taller) and therefore stronger cloud
tops of the thunderstorms. In late May, the ground of the
Southwest and northern Mexico is hot with no thunderstorms, the Sierra Madre
Occidental Mountains of Northwest Mexico show up in dark blue. By June
5, as the water temperature of the Gulf of California rises, moisture is
pulled on shore and a few thunderstorms are seen in the lower part of the
image near the mountains. By mid June, daily development of
thunderstorms is seen over Mexico, while Arizona remains dry.
These storms, however, are pushing moisture northward towards the Southwest.
By late June into the first of July, thunderstorms are forming in Sonora
just south of the Arizona border. By July 8, the Monsoon has become
established over Arizona with thunderstorms developing daily, including a
major outbreak on July 19th.
| What Am I Looking at?
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Loop of IR Satellite Images May 25-July 19 |
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Last Update:
Sunday August 03, 2008 14:48:36 -0700 |
Internet Weather Service
Tucson, AZ 85743
Page last modified:
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