are fairly common during the monsoon in Arizona. Strong downburst
winds, flash flooding and
cloud-to-ground lightning are the most frequently observed thunderstorm
phenomena but hail, blowing dust and
even an occasional small tornado can also be observed.
Associated with Severe Thunderstorms.
The Type I - pattern
is the "classic" monsoon pattern. The streamlines depicted below
are for the mid-levels or about 20,000
feet above sea level. A broad high is positioned across the southern
half of the United States with easterly flow
located over portions of Texas, New Mexico, Arizona and northwest
Mexico. Easterly flow in Mexico means lots
of thunderstorms in the mountain foothills. High pressure over the
southwest United States almost guarantees that
the surface based thermal low is deepening (refer to the Gulf Surge
The Type II -
pattern is associsted with a high amplitude ridge over the western
United States. The streamlines indicate that easterly to northeasterly
flow is located across Arizona with this pattern. Once again
all the elements are there, easterly flow over Mexico for lots of
storms and intense heat over the west building the surface
based thermal low. On occasion a weak cool front will move into
Arizona from the east with this pattern and actually bring
moisture from the High Plains into Arizona.
The Type III -
pattern is also associated with active periods of the monsoon. The
elements are similar to the previous
two pattern types easterly flow over northwest Mexico with hot temperatures
across Arizona by virtue of
there being a high pressure system overhead. Typically below 20,000
feet the flow over Arizona is light and variable
with this pattern making it fairly easy for Gulf Surges to enter
Arizona from the south.
Why are Some
is a concentrated area of strong winds in the vicinity of a thunderstorm.
When conditions are right downburst winds can attain speeds of over
100 mph! This is more than enough to lift a roof, roll a mobile
home or bring
down power lines. luckily downburst with wind speeds greater than
100 mph are rare. More typical are wind speeds
of 40 to 50 mph. The main ingredients for a severe downburst (58
mph or greater) are an above average high temperature,
low relative humidity and above average total moisture in the layer
underneath the developing thunderstorm.
When these three ingredients combine severe downbursts are usually
When the atmosphere
is extremely moist any individual storm is capable of producing
localized flooding. This occurs
when very moist air ascends, forms a thunderstorm and produces an
extremely intense rain shaft. This is a fairly
common occurrence in southeast Arizona during the monsoon.
A flash flood
producing thunderstorm is usually more organized than the garden
variety heavy downpour. Flash flood producing storms can be classified as mountain slope or desert valley flash flood storms.
In the mountain
slope storms moisture laden flow is upslope towards the mountains.
The moist flow destabilizes as it
moves up the mountain slope forming a series of thunderstorms. One
thunderstorm cell after another will form and move
over the same area. This results in rapid runoff which can travel
many miles beyond the direct influence of thunderstorm activity.
In the desert
valley flash flood storm a different mechanism is at work. Like
the mountain slope storm there is moist low
level flow. As thunderstorms initiate within this moist flow thunderstorm
outflows form a boundary at the surface.
This boundary acts as a focusing mechanism for further thunderstorm
development. Typically new cell developemnt will be toward the low-level
inflow with a succesion of new storms forming and moving over the
Some of the most damaging and severe flash floods in Tucson have
been a direct result of the desert valley flash
flood type storm.
Of course with
any strong thunderstorm frequent cloud-to-ground lightning is common
and when conditions are right
severe downburst wind and flash flooding can accompany a thunderstorm
outbreak. Large hail, blowing dust and
even small tornadoes all can result when the atmosphere over southeast
Arizona becomes very unstable.
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