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DESERT FLASH FLOODS AND THE SOUTHWESTERN MONSOON

Every year, the southwestern United States is affected to one degree or another by a phenomenon known in meteorological circles as the Southwestern Monsoon. Also known by the names Mexican Monsoon and Arizona Monsoon, as well as the more general term monsoon season, this is a weather pattern that brings occasional heavy rainfall to the deserts of the Southwest and even as far north sometimes as the mountains of Utah and Colorado and the Great Plains of Nebraska and South Dakota.

The term monsoon actually derives from the Arabic word mausim, which simply means season in that language. (This makes the commonly-used term monsoon season redundant, but we digress...) The term is commonly misused or misunderstood in Arizona, for the monsoon applies to a rather specific weather pattern, and not just any rainstorm or thunderstorm, as is usually supposed..

HOW THE MONSOON BEGINS

What the monsoon actually refers to is a seasonal large-scale atmospheric shift in winds, and the types of weather that this large-scale annual shifting brings. In Arizona, there are two very distinct parts to summertime. The first half of summer, which begins in the latter part of April, and definitely lasts all of May and June, is very hot and dry. It rarely or never rains in most locations in Arizona during these months, especially not at lower and moderate desert elevations. Even if it does rain, the accumulations are rarely significant. Winds prevail mainly from the west or northwest, and are very dry, resulting in low relative humidity, sunny skies, and blistering temperatures as the sun reaches its annual zenith during these months.

The second half of summer begins when the so-called monsoon arrives. More accurately, it could be called the arrival of a monsoonal shift, as there are really two monsoons: the dry and the wet. The winds begin to shift 90 degrees or more from their prevailing flows, and start to arrive from the southeast or even the east in some instances. Subtropical and tropical moisture arrives along with this shift in the wind patterns, which helps touch off torrential thunderstorm complexes when combined with the high levels of solar radiation and daytime heating that Arizona receives. This can result in periods of heavy rainfall, and even in flash flooding when thunderstorms drop enough water fast enough to cause this dramatic event.

It should be noted that not every summer thunderstorm is a monsoonal one. There are plenty of summer storms that pop up on average summer afternoons as a result of water that has been evaporated off of lakes and rivers and out of the moisture transpired by forests and shrublands and agricultural crops in Arizona. The Southwestern Monsoon applies to very moist air that has been drawn up from either the tropical Pacific Ocean off of southern Mexico or off of the subtropical Gulf of Mexico, or, in some cases, from both sources simultaneously. Monsoonal moisture arrives from the southeast, south, or southwest. Generally any moisture associated with more temperate waters off of the Pacific Ocean near California is termed extratropical, and is merely a part of the "normal" prevailing westerly winds.

Water vapor that originates in cooler Pacific waters near California may move inland and touch off thunderstorms, but storms originating from this dynamic are not considered to be monsoonal in nature. They are generally just called summer thunderstorms, not monsoonal thunderstorms. One other difference is that these so-called summer thunderstorms are usually not great rainfall producers. They lack the widespread, deeply-saturated air from the tropics that results in torrential rainfall, especially over the lower deserts of the state. Summer thunderstorms are often restricted to higher altitudes and are common producers of "dry lightning", or lightning without associated rainfall, which can result in the ignition of forest fires all across the West. Monsoonal storms usually have lots of rain associated with them and donít produce dry lightning as often.

As a final note, the term monsoon never applies to winter rains, which almost always originate in cool to cold Pacific waters, arrive from the north and west most often, and are gentle and frontal in nature, rather than turbulent and violent like thunderstorms tend to be. Winter frontal storms account for the other half of Arizonaís precipitation, although they tend to get much less press than the intense storms of summer.

A late-season monsoon thunderstorm makes an encore appearance in September 1999 over the Joshua tree forest of the Hualapai Mountains near Yucca, Arizona

Rainfall patterns

The summer monsoon is very important to the ecology of the southwestern US, particularly the Chihuahuan and Sonoran Deserts, but also to the interior chaparral and montane ecosystems as well. Everything benefits from the occurrence of rainfall in this largely semiarid to arid region, and many species of animals and plants have adapted to time their reproduction and migration around this yearly cycle.

The Southwestern Monsoon is primarily a Mexican phenomenon, with only the southernmost portions of New Mexico, Arizona, and West Texas receiving regular influence from the rains. Most of northern Mexico, however, gets approximately two-thirds of its annual rainfall in the months of July, August, and September. The monsoon tends to begin in the dry tropical regions of southern and central Mexico near Acapulco and Mazatlan in mid-June, and works its way progressively northward over the course of several weeks until it begins to strongly influence the weather of southeastern Arizona by about the Fourth of July. The monsoon tends to arrive on or about July 5 in Tucson, July 7 in Phoenix, and about July 9 in the northern parts of Arizona up by the Grand Canyon. Of course these dates are approximate and can vary some, but long-term record-keeping has shown that the monsoon is remarkably reliable at arriving within two or three days of these dates for the aforementioned locations in Arizona most years.

The monsoon is also much more potent in northern Mexico than it is in Arizona on average. Rains in Sonora and Chihuahua, Mexico, may occur daily for several months, especially in mountainous areas near the humid coasts which favor the development of powerful storms. Many locations in the mountains of the Sierra Madre, in the Mexican state of Sonora, will receive six to ten inches of rainfall in July alone with another five to eight inches in August and September. As noted earlier, this represents two-thirds to three-quarters of the annual average total rainfall for these regions. As such, the importance of the Southwestern (Mexican) Monsoon to the people, plants, and animals of northern Mexico cannot be understated.

Monsoonal activity tends to spread thinner and become less frequent and less reliable as one moves further north. Tucson usually receives about three inches of rain in July, with another three inches possible in August and September. By comparison, Phoenix, which lies another 100 seemingly inconsequential miles further north of Tucson, might see only three to four inches of rain for the three months that the monsoon is usually present. And so it goes on down the line until the monsoonal influence is absent altogether for most of the year in locations much beyond central or northern Arizona.

SAGUARO CACTUS AND SUMMER RAINFALL

The Sonoran Desert relies heavily upon the arrival of the monsoon rains on both sides of the border. Coming as they do on the heels of the most stressfully hot and dry time of the year in May and June, the occasional rains are critical to the survival of most species of Sonoran creatures. In fact, the very symbol of the desert, the saguaro cactus, requires monsoon rain for its seeds to germinate and carry on the next generation of plants. The range of saguaros is largely determined on the west and northwest by the presence or absence of adequate monsoonal rainstorms. In fact, the saguaro times its flowering to occur during the dry and hot portion of the year in May and June so that the fruits will ripen their seeds in time for the arrival of the first thunderstorms creeping up from Mexico in July. If the storms are generous with rainfall and frequent enough to permit the little cactus seedlings to germinate and survive their first few weeks of life, then there is a chance that they can survive into adulthood to propagate the species.

The Chihuahuan Desert of New Mexico and West Texas is very monsoonal in nature, with the large majority of the six to ten annual inches of rain falling during the months between July and October. So why donít saguaros live there? The answer lies in the fact that the Chihuahuan Desert is a high-altitude desert located primarily east of the Rocky Mountains, which makes it more prone to sudden Arctic cold snaps that can drop the temperatures into the lethal single digits for small saguaros. The Rocky Mountains tend to act as a barrier to extreme cold fronts arriving from the north in Arizona, and this combined with lower altitudes overall make for a suitable ecological niche for saguaros. Saguaros are found almost exclusively in Arizona and Sonora, Mexico, aside from a few hundred naturally-occurring plants found just across the Colorado River no more than a few miles into California.

And what about the Mojave Desert? The Mojave is lacking in saguaros for other reasons. The portions of the Mojave Desert that are located further north and at an average higher altitude also echo the reasons that saguaros are missing from the Chihuahuan Desert with respect to low winter temperatures. But there are large portions of low-elevation Mojave Desert in California and southern Nevada that donít grow saguaros where temperatures and soils are generally otherwise suitable. The reason here is because the monsoon rainfall does not occur with enough abundance or regularity to support saguaro reproduction. The Mojave Desert receives almost all of its rainfall in the wintertime from the cold Pacific fronts mentioned earlier, when it is too cold for saguaro seeds to germinate. Summers are very hot and almost rainless in much of the Mojave Desert most years, although a particularly strong monsoonal impulse will push storms north- and westward to produce occasional soaking rainfall in summer. But this is rare (not more than once every year or so) and this infrequent, spotty, and unreliable summer rain will not support vulnerable saguaro seedlings for their first critical few months often enough to allow the species to colonize the lower and warmer areas of the Mojave Desert, as they might otherwise do.

For some excellent photographs of the monsoon season thunderstorms, including fascinating satellite imagery of what exactly causes the monsoon season to initiate in June in Mexico and then begin shifting north to the US in July, visit the Arizona Monsoon page at http://www.psiaz.com/polakis/monsoon/monsoon.html. Another good site is the National Weather Serviceís Monsoon Page from the Phoenix office at http://www.phx.noaa.gov/general/monsoon/. There are a number of other interesting sites with good photographs and descriptions of the Southwestern Monsoon, too many to mention here, but all worth a look for the intriguing pictures and interesting data on rainfall and temperature shifts.

FLASH FLOODING AT D:FR

In the first week of September 2002, a surge of monsoonal moisture from the southeast made it all the way north and west to the area in which D:FR is located. The high humidity actually arrived in the area late in the afternoon and early evening hours on September 5, with some moderately-strong rain showers occurring on Sept 6. However, the main rainfall event did not occur until the early morning hours of Saturday, September 7, 2002, when an extra reinforcing shot of very wet tropical air associated with the former Hurricane Hernan in the Pacific Ocean southwest of Cabo San Lucas, Baja California, Mexico touched off widespread drenching thunderstorms and torrential downpours over the deserts of western Arizona.

 

Destination:Forever Ranch received a total of 2.1 inches of rain in less than two hours that morning. This rainfall was particularly well-appreciated because it marked the lone precipitation-producing event of the 2002 monsoon season in this part of northwestern Arizona. Although there had been several other promising-looking times when the humidity skyrocketed and the cloud cover thickened earlier that summer, the hoped-for rains failed to materialize, thereby underscoring the sporadic nature of the monsoon season this far west and north. It does bear mentioning that the months between April 2001 and September 2002 saw little to no rain at all in this area. This is part of an ongoing cycle of serious drought in the southwestern states, which in turn is part of a larger drought pattern afflicting most of the West and Great Plains states in the 1990's and early 2000's thus far. The Southwestern Monsoon has not made a great showing for a number of years in this area, but fortunately the flora and fauna are adapted to ride out such difficult circumstances until conditions improve, which they eventually will do again.

Since this September 7 rainfall event was happening in a year that was seriously dry even by reduced desert standards, General Manager Jan Emming wanted to see how the desert would respond to the sudden influx of a large quantity of water. Flowing water is a real luxury in the desert, even when it arrives in the form of flash floods that last less than an hour. Still, it had been so long since any real precipitation had occurred, that he took his camera and ventured out into the driving torrents to capture the following photographs. The photos were taken from the mid-point of the storm at about 9 AM onwards to about 10:30 AM, when the last raindrops of this event fell, leaving the desert to heave a sigh of relief for the long-delayed gift of water. We hope that this photographic series will help illustrate the power and drama of rainfall in the desert, while increasing human appreciation for the cycle of life that sustains us all. (click on the images to view a larger version, and then use your browser's back button to return here)