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Welcome to Severe Weather Patterns Information Page

Typical Patterns for Severe Weather during the Arizona Monsoon



Back to Monsoon Basics Page




Severe Thunderstorms in Arizona


Stormy Weather!

Severe thunderstorms 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.

Weather Patterns 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 discussion).


This graphic shows the TYPE 1 upper air pattern for severe weather is AZ


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.

This graphic shows the TYPE 2 upper air pattern for severe weather is AZ


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.

This graphic shows the TYPE 3 upper air pattern for severe weather is AZ


Why are Some Storms Severe?

A downburst 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 the result.

3-panel graphic showing evolution of a downburst

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.
 

Graphic show heavy downpour and lightning

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 same area. Some of the most damaging and severe flash floods in Tucson have been a direct result of the desert valley flash flood type storm.
 

graphic show different types of moist upslope flow

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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