The University of Arizona

August 2011 Vol. 1 Issue 3 | CLIMAS

August 2011 Vol. 1 Issue 3






PDF Version of August 2011 Monsoon Tracker



Monsoon storms deliver blinding dust storms, powerful winds, and raging floods that can wreak havoc on people and property. The intense rains also snuff out fires, ripen pastures, temper temperatures, and breathe life into dry landscapes. This summer the monsoon will play a vital role in reversing or amplifying widespread drought conditions in the Southwest and quelling fires.

To help the Southwest stay informed of observations, insights from experts, and cutting-edge science related to the monsoon and climate, the Climate Assessment for the Southwest will publish the Southwest Monsoon Tracker each month through September. This issue will feature a summary of the monsoon since it officially began on June 15.

Monsoon Summary: June 15 - August 3

The typical monsoon pattern, in which drier conditions in western Arizona progressively become wetter to the east, has flip-flopped through the first half of the monsoon season. The culprit? The position of the subtropical high, or the monsoon ridge, which has hovered too far to the east to deliver copious rains to most of New Mexico. This extensive dome of high pressure has generally extended from the East Coast to eastern New Mexico, causing winds over New Mexico to waft generally from the east instead of the south.

The southeastern corner of New Mexico has fared the worst, receiving less than 50 percent of average in the last 30 days, or between 1 and 3 inches below average—the image above excludes several days of monsoon activity prior to July 5, but theses events do not substantially change the rainfall pattern (Figure 1). This area also has been the warmest, compounding the effects of the exceptional drought in this area. Roswell, for example, already has broken its record for the number of days with temperatures of 100 degrees Fahrenheit or greater.

The position of the ridge has effectively reduced the number of favorable stormy days, and when storms have formed, they generally have been spottier than normal, according to the National Weather Service in Albuquerque. Arizona, however, has been wetter than New Mexico, mostly because it has profited from three gulf surges related to tropical storms and hurricanes Arlene, Dora, and Don. In July, Phoenix received 1.41 inches of rain, 0.41 inches above average, but nearly all of that came in two gulf surge dollops. The location of the high pressure system, however, blocked these surges from flowing past eastern Arizona. In summers with copious rain, the high pressure is generally positioned over the Four Corners region, sucking moist air from the Gulfs of California and Mexico. This year the moisture source has been predominantly from the Gulf of California and has delivered high humidity. Dew points, a measure of humidity, have been persistently high for most of the region—there has been only one monsoon break in which dew points dipped below 54 degree F in southern and central Arizona for an extended period.

Monsoon Conditions

Spotty monsoon rains have not helped alleviate drought conditions in most of Arizona and New Mexico, where the total area covered by all drought categories (abnormally dry conditions is not a drought category) is 61.3 and 100 percent, respectively, and remains virtually unchanged from one month ago (Figure 2).
Southern New Mexico and southeastern Arizona, which have received below-average rain, continue to be classified with exceptional drought, which is defined as a drought that occurs once in every 50 years.
Even with above-average rainfall during the remainder of the monsoon season, drought conditions likely will only improve by one drought category.
Southeastern New Mexico has been the hottest and driest part of the state; less than 1 inch of rain, on average, has fallen in this region—as    much as 3 inches below average—while temperatures have been 4−8 degrees F above average.

Crop and pasture lands in nearly all of New Mexico fall under moderate to exceptional drought classifications (Figure 3); the conditions for Arizona and New Mexico are similar to one month ago.
Temperatures have been 2–8 degrees F above average in the eastern half of New Mexico, which reflects the strong correlation between precipitation and temperature—rainfall typically lowers temperature.
New Mexico experienced the second warmest July on record, which spans 117 years. Texas and Oklahoma set records, while Arizona’s average temperature was the 38th warmest.
The dry winter, combined with storms that generated little precipitation and strong winds, set the stage for several large dust storms that rolled through central Arizona.
Currently, eight fires are burning in Arizona and seven in New Mexico; most of these are relatively small, having charred less than 900 acres as of August 4. Lightning ignited all of these fires except one in New Mexico.

The Final Word

The one-month forecast for August issued by the NOAA-Climate Prediction Center (CPC) calls for slightly increased chances for below-average precipitation in the southeastern corner of New Mexico and equal chances for above-, below-, or near-average rainfall for the rest of Arizona and New Mexico.
The CPC has issued a La Niña Watch, which means that conditions are favorable for the development of a La Niña event within the next six months. The La Niña event this past winter upheld the typical precipitation pattern in the Southwest by delivering below-average rainfall, which set the stage for the largest fire season on record and widespread drought conditions. A back-to-back La Niña could accentuate the acute drought impacts already gripping the region and place additional urgency on a wet second-half of the monsoon season to help minimize the impacts of a potentially dry winter.
While some areas received above-average rainfall through August 3, the monsoon thus far has been underwhelming for many regions. In southern Arizona, and likely for other regions, weak winds aloft and warm temperatures in mid-altitudes in the atmosphere combined to help impede convective activity and hinder storms from blowing off high elevations and into valleys.
The general position of the subtropical high thus far has limited moisture incursions into New Mexico from the south and blocked moisture surges originating the Gulf of California from wafting in the region.