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Summer Severe Thunderstorm
Patterns in Arizona. PDF
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In the
1990s, three general, large-scale weather patterns were identified which
are favorable for producing severe thunderstorm and flash floods in central
Arizona. (McCollum 1993, Maddox et al. 1995). Because of the lack of a dense,
statewide severe weather spotter network, the research focused on mainly
the Phoenix Metro Area. It also focused on just July and August from 1978
to 1990. Since then, a much better spotter network and rapid population
growth has greatly expanded our severe weather database, particularly in
the southeast and along the Mogollon Rim. The new data has confirmed that
these three patterns support severe thunderstorm and flash flood outbreaks
over much of the state. However, the additional data suggest that a fourth
pattern, which tends to occur late in the monsoon season, produces its share
of damaging winds, hail and flash flooding as well. |
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Type
I: Southern Plains/Four Corner High |
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This is
the most common severe thunderstorm pattern for southeast and south central
Arizona, especially early in the monsoon. In this situation the monsoon
ridge sets up over the southern Plains and extends west to the Arizona-Utah
border. A secondary high usually develops near the Four-Corner region. When
this happens, mid level temperatures across southern Arizona cool, low level
moisture increases from the south or east, and winds between 10,000 and
20,000 feet increase out of the east. This causes thunderstorms to tilt
slightly, and allows them to maintain themselves for longer periods of time
while organizing into lines or clusters. If the lower levels of the atmosphere
are rather dry, straight line winds and dust storms are a major concern.
If the lower levels are moist, flash flooding becomes a problem as well. |
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Initially,
thunderstorms on Type-I days form on the mountains and spread east-to-west
or southeast-to-northwest. Thunderstorms on the Mogollon Rim and in the
White Mountains tend to remain where they develop, while the storms in the
mountains of southeast Arizona or northern Sonora tend to move into the
valleys and eventually the low deserts. As these storms move progressively
farther to the north or the west, they typically encounter a more stable
atmosphere and dissipate. When a Gulf Surge is underway, though, the atmosphere
remains unstable as the storms move into the lower deserts. In these instances,
storms may continue to travel all the way to the Colorado River Valley. |
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Type
I: Southern Plains / Four Corner High |
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Type
II: Great Basin High |
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This severe
thunderstorm pattern is less common, and tends to occur during the height
of the monsoon. In this situation the monsoon high migrates unusually far
northwest into the Great Basin of southwest Utah or southern Nevada. Over
the eastern U.S., an unusually deep upper level trough develops which sometimes
pushes a cold front south through the Plains and west toward the Arizona-New
Mexico border. The clockwise circulation around the upper level high causes
winds between 10,000 and 18,000 feet increase out of the northeast over
Arizona. |
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As the
thunderstorms develop on the mountains, the Mogollon Rim readily forces
them to organize into squall lines, which are then pushed southwest by the
winds aloft into the deserts. The northeast winds aloft usually bring drier
air into Arizona, so if the drying is deep enough, the thunderstorms may
dissipate before moving very far away from the higher terrain. However,
if there is only drying aloft and low level moisture remains plentiful,
the downdrafts associated with these lines of storms can become large and
severe. Areas most susceptible to Type II events are those immediately downwind
from the Rim or White Mountains of east central Arizona, including: the
Phoenix Metro Area, the Gila River Valley, and the valleys of Yavapai County.
The changing wind direction and speed with height also helps to sustain
the thunderstorms for even longer periods than in Type-I patterns, which
can allow them to persist well into the night as they move southwest through
Tucson and the Colorado River Valley. |
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Type
II: Great Basin High |
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Type
III: Trapping High |
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This pattern
is quite different than the other two in that the monsoon ridge is weaker
and suppressed father to the south - sometimes extending along the U.S.-Mexican
border. The ridge will sometimes break into two separate centers with one
over south Texas and the other over northern Baja. If this occurs either
in June or September, the ridge placement tends to block moisture coming
north from Mexico. However during monsoon peak, moisture still finds its
way into Arizona from the south and east. Meanwhile, upper level disturbances
can move into the region from several different directions, and either slow
down, or become trapped within the ridge and stall. The presence of a weak
upper low keeps temperatures aloft relatively cool, and the entire atmosphere
unstable. |
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Winds
aloft in Type III patterns are usually very light. Though thunderstorms
are not particularly well organized due to the lack of wind shear, they
tend to move slowly or remain relatively stationary. Moisture becomes trapped
over the region, which only causes more slow-loving thunderstorms to develop.
This rather stagnant situation will end when either the high strengthens
and causes mid level temperatures to warm, or winds aloft increase. |
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This pattern,
which tends to develop most often in late July or August, can result in
flash flooding and isolated. In fact, the atmosphere can remain so moist
and unstable that thunderstorms may not follow the typical diurnal pattern
of developing in the afternoon and fading a couple of hours after sunset.
The most notorious and recent example of what this pattern can do occurred
between July 26 and August 1, 2006. An upper level disturbance stalled between
breaks in a weak monsoon ridge along the International Border. Widespread
flash flooding plagued much of Arizona for several nights in a row which
resulted in millions of dollars in damage. |
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Type
III: Trapping High |
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Type
IV: Transitional |
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This pattern
does not need the monsoon itself to generate severe weather in Arizona.
It tends to develop sometime in late August or September, and usually acts
on moisture that was transported into Arizona during the monsoon season.
This pattern is called "transitional" because winds aloft shift
from the tropical easterlies back to the southwest or west. This typically
happens as the subtropical high weakens and shifts southeast into northern
Mexico or the Gulf of Mexico, and a trough of low pressure develops near
the West Coast. This trough is sometimes accompanied by a weak surface cold
front, which helps to organize thunderstorms development. |
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Surface
winds ahead of these upper level troughs usually remain out of the south
or southeast, while upper level winds shift to the southwest or west. The
resulting wind shear can cause squall lines, or rotating, supercell thunderstorms
to develop. This pattern is actually similar to the conditions that trigger
severe weather in the Great Plains during the spring and summer. Tornadoes
are uncommon in Arizona, but this is the one pattern most likely to support
stronger ones, especially from the Phoenix area north into the Rim Country
and along the Utah border. Large hail can also be a problem in these situations,
in addition to the damaging winds. Type-IV patterns can also cause flash
flooding if the front moves slowly, or if the front taps into a tropical
system well to the south. Once the trough and associated cold front passes
through Arizona, dry westerly flow at all levels of the atmosphere usually
overspreads the region. After a Type-IV event, the weather usually turns
quiet across Arizona for several days, and may even signal the end of the
monsoon. |
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Type
IV: Transitional |
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References: |
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Maddox, R.A., McCollum, D., and Howard, K., 1995: Large-scale patterns
associated with severe summertime thunderstorms over Central Arizona.
Wea. Forecasting, 763-778.
McCollum, D.M., 1993: Synoptic-scale patterns associated with severe
thunderstorms in Arizona during the summer monsoon. M.S. thesis, School
of Meteorology, University of Oklahoma, 166 pp.
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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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