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Reflections from the Backcounter
Long time Arizona residents are well acquainted with the summer monsoon season. Often used incorrectly, this term is used to describe the afternoon thunderstorms that predictably occur during late July through September in the Sonoran desert. These storms can be preceded by huge and violent duststorms and are often accompanied by severe wind. True desert dwellers have a healthy respect for the lightning, thunderstorms and sudden flash floods that can create serious danger in a short period of time.
So, last night...we had the Mother of all Monsoon storms. The lightning was incessant and the storm was accompanied by hail that was blowing sideways into our windows with such force, I expected the glass would break. As it turned out...the winds in downtown Phoenix reached speeds of 100 mph. My sister is a property manager and has reached the varsity team of property managment as she manages huge high rises...unfortunately, her building was at ground zero and this storm caused hurricane like damage on this building. She is just now returning home after more than 18 hours on the job trying to restore the building to a usable state.
This topic is especially pertinent this weekend as we await the U.S. landfall of Hurricane Gustav. My prayers are with the people of Louisiana and the gulf coast states.
For those of you who truly want to know more about the monsoon phenomenon read the following description from the ASU website:
The Arizona Monsoon is a well-defined meteorological event (technically called a meteorological 'singularity') that occurs during the summer throughout the southwest portion of North America. During the winter time, the primary wind flow in Arizona is from the west or northwest—from California and Nevada. As we move into the summer, the winds shift to a southerly or southeasterly direction. Moisture streams northward from the Pacific Ocean and the Gulf of Mexico. This shift produces a radical change in moisture conditions statewide.
Such a change, together with daytime heating, is the key to the Arizona monsoon. This wind shift is the result of two meteorological changes:
The movement northward from winter to summer of the huge upper air subtropical high pressure cells, specifically the so-called Bermuda High (H).
In addition, the intense heating of the desert creates rising air and surface low pressure (called a thermal low) in the Mohave (L).
These two features combine to create strong southerly flow over Arizona. The southerly winds push moisture north-ward from Mexico. The exact source region for the moisture of the Arizona monsoon is unknown. Researchers have proposed the Gulf of Mexico and/or the Gulf of California as the source regions but conclusive evidence has so far been elusive.This has lead to the creation of large data-collecting efforts and research programs such as SWAMP, the Southwest Area Monsoon Project .
By the way, the term "monsoons" as in "when the monsoons arrive ..." is a meteorological no-no. There is no such beast. The word should be used in the same manner that "summer" is used. Consequently, the proper terminology is "monsoon thunderstorms" not "monsoons."
Monsoon thunderstorms are convective in nature. By that, we mean that the thunderstorms are powered by intense surface heating. In addition, strong moisture influx into Arizona is also required. The operational criterion for the onset of "monsoon" conditions used in Arizona is "prolonged (3 consecutive days or more) period of dew points averaging 55°F" or higher." There is nothing magical, however, about 55°F. It originally was linked to the total amount of water in the atmosphere above the weather station (a precipitable water amount of 1", a quantity thought to be necessary for convective thunderstorm activity). In general, for Phoenix, the temperature limits for the production of monsoon thunderstorms are 100° to 108°F with the optimum temperature being about 105°F. Temperatures needed to produce Tucson's thunderstorms are somewhat lower.
The Arizona monsoonal circulation does not produce thunderstorms every day during the months of July-September but rather occurs in a pattern that has "Bursts" and "Breaks". According to climatologist Andrew Carleton:
"Burst": a movement into Arizona of a weak trough in the upper level westerlies (normally during summer these winds are far north of this location) which spreads upper level cold air into the region. In lower levels, during a "burst", there is strong surface heating and strong southerly or southeasterly transport of moisture into Arizona. This creates intense atmospheric destabilization and leads to strong widespread thunderstorm outbreaks.
"Break": an enhanced ridging of the upper level Bermuda and North Pacific subtropical high pressure systems which acts to stabilize the atmosphere and thereby cutoff widespread thunderstorm activity. Our own meteorologists suggest that a break usually occurs when the subtropical ridge re-develops over NW Mexico and drier air spreads into
1 comment:
Some of my family lives in Arizona (Sierra Vista) and have often spoken of the monsoons. But I never quite knew what they were. Thanks for your vivid descriptions! They sound a little like our New England Nor'Easters! Although, we get blizzards with those. ;)
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