Forecast Center

September/October 2003


This article is a courtesy copy placed on the author's website for educational purposes as permitted by written agreement with Taylor & Francis. It may not be distributed or reproduced without express written permission of Taylor & Francis. More recent installments of this article may be found at the link which follows. Publisher's Notice: This is a preprint of an article submitted for consideration in Weatherwise © 2003 Copyright Taylor & Francis. Weatherwise magazine is available online at:

PART ONE: The Puzzle

It is noon in the southwestern Desert in July. A unique and very important weather phenomenon is about to occur, one which brings a large percentage of Arizona's annual rainfall. Beginners may want to consider this hint: fronts will not be found on this issue's chart, however dewpoint temperatures will play an important part in the solution.

Draw isobars every four millibars (992, 996, 1000, 1004, etc.) using the plot model example at the lower right as a guide. As the plot model indicates, the actual millibar value for plotted pressure (xxx) is 10xx.x mb when the number shown is below 500, and 9xx.x when it is more than 500. For instance, 027 represents 1002.7 mb and 892 represents 989.2 mb. Therefore, when one station reports 074 and a nearby one shows 086, the 1008 mb isobar will be found halfway between the stations.

Click to enlarge

* * * * *

Scroll down for the solution

* * * * *

PART TWO: The Solution

Although America's southwest deserts may conjure up images of saguaro cactus, blue sky, and relentless heat, it might be a shock to hear that it is home to monsoon weather. It's a phrase that evokes images of torrential storms in India and endless weeks of rain in Vietnam. Surprisingly, North America's Mexican monsoon is not too different from its Asian cousins.

The solution shows the Mexican monsoon as it first began affecting southern Arizona on June 12. One of the keys to the monsoon puzzle is the semi-permanent thermal low that is usually found over the desert Southwest during the summer months. Caused by intense solar heating, it is seen on the solution stretching from western Mexico to Nevada. The weak wind flow between this thermal low and higher pressures in Mexico are what allows tropical moisture to stream northward. The moisture originates from the Gulf of California and even the Gulf of Mexico, and dewpoints in Arizona may climb into the 50s and 60s as shown in the example. This rich moisture, combined with the very strong surface heating and orographic lift, produces widespread thunderstorms.

Considerable debate has surrounded the origin of monsoon moisture. Up until the 1980s it was believed that nearly all moisture was piped in by the Bermuda High in the Atlantic Ocean, bringing Gulf of Mexico moisture northwestward across northern Mexico in the mid-levels of the atmosphere. However an important study in 1993 indicated that evaporation from the Gulf of California, whose surface temperatures climb well into the 80s, could be an extremely important source. This air mass, combined with recycled moisture from western Mexican thunderstorms along the mountain ranges, are thought to be the primary sources of moisture.

Normally, an ample surface network of surface and rawinsonde stations would be the key to forecasting the monsoon. Unfortunately, as can be seen on the puzzle map, these simply do not exist in northern Mexico, and the isobars do not show any distinct patterns. The surface chart in itself is of little help except to identify the surface dewpoints that exist. Forecasters must fall back on the time-tested technique of continuity. Dewpoint temperatures are carefully tracked from day to day at key stations in Arizona and northern Mexico. In addition to trend analysis, pattern identification techniques are also being used which help forecasters foresee the consolidation of ingredients that fuel monsoon events.

But for the rest of us, Arizona monsoon thunderstorms are welcomed by Arizona weather buffs as a break from summer's monotony. They're also a favorite among many storm chasers. Here, the atmosphere is not tainted by haze, treelines, and blankets of low cloud. Framed by Arizona's timeless landscape, monsoon storms unfold crisply and beautifully in all colors of the spectrum.

Click to enlarge

©2003 Taylor & Francis
All rights reserved