The University of Arizona

October 2011 Vol. 1 Issue 5 | CLIMAS

October 2011 Vol. 1 Issue 5






PDF version of October 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 publishes the Southwest Monsoon Tracker from June to September. This issue features a summary of the entire monsoon, from June 15 to September 30.

Monsoon Summary: June 15 - September 30

The 2011 monsoon began and ended with a bang but was lackluster in between. While many regions experienced near-average rainfall, a few periods of widespread and copious rains padded the statistics, particularly in mid-September, turning an otherwise dry summer into an average one (Figure 1). But after a historically dry winter and a paucity of precipitation forecast for upcoming months, the Southwest will take rainfall in any way it is delivered.

The most vigorous monsoon activity came between September 9 and 16, when many parts of both states received more than 200 percent of average rainfall. A low pressure system parked over the region and cool air wafted into the upper atmosphere, creating atmospheric instability that combined with ample moisture to generate extensive and intense storms. Strong winds aloft also helped blow the storms off the mountains and into the valleys, causing widespread rainfall that generally was absent during the summer. This one-week flare up might have saved the 2011 monsoon season from being labeled a dud. Even with these rains, most regions experienced between 75 and 100 percent of average rain between July 1 and September 30. Other areas, however, recorded above-average rainfall, including southwest Arizona, which received the brunt of monsoon activity. Tucson, for example, notched a total of 8.62 inches, which is about 3 inches above average and is the 10th wettest monsoon since record keeping began 117 years ago. In other cities, monsoon storms were few and far between. Rain in Yuma totaled 0.76 inches, which was 0.36 inches below average. Phoenix measured 1.6 inches of rainfall–1.16 inches below average. The monsoon delivered 4.13 inches of rain in El Paso, or 0.87 inches below average. And in Albuquerque, rains totaled only 1.72 inches—2.4 inches below average—during the fifth driest monsoon in the city’s history. The driest part of the Southwest covered the southeast corner of New Mexico, prolonging an arid spell that began last winter.

Several conditions helped suppress the monsoon this summer. In Arizona, despite the presence of ample moisture, a relatively warm upper atmosphere and weak winds aloft generally inhibited convection and the movement of the storms off the mountains. Most of New Mexico, on the other hand, had low moisture. This can be partially blamed on the location of the subtropical high, or monsoon ridge, which was centered over Oklahoma and Texas—too far east to regularly steer moisture into southeast New Mexico. The position of the ridge is a cause and consequence of exceptionally dry conditions in Texas and southeast New Mexico.

Monsoon Conditions

Drought conditions remain exceptional in most of southeastern and southwest New Mexico, according to the U.S. Drought Monitor. All but about 4 percent of New Mexico is experiencing moderate or more severe drought conditions. In Arizona extreme drought covers about 14 percent of the state, with southeastern Arizona and parts of the Colorado Plateau experiencing the most severe drought (Figure 2).

The pattern and severity of drought conditions did not change substantially in New Mexico between June 14 and September 27. At the beginning of the monsoon, extreme and exceptional drought occupied about 45 and 23 percent of the state, respectively. On September 27, about 35 and 34 percent of New Mexico was classified with exceptional and extreme drought, respectively.

The total area occupied by moderate drought or a more severe drought category increased by about 14 percent in Arizona from June 14 to September 27. In the southeast corner, drought improved one category from exceptional to extreme, while on the Colorado Plateau drought worsened two categories, moving from moderate to extreme.

Temperatures in September generally have been 2–4 degrees Fahrenheit above average in southern Arizona and New Mexico. New Mexico experienced the warmest June–August period in its history, and both maximum and minimum temperatures were 2–4 degrees F above average in the eastern half of the state. Arizona experienced the 11th warmest June-August on record (data for June–September is not yet available). Phoenix experienced the warmest monsoon on record.

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.

The U.S. Department of Agriculture has doled about $15 million to livestock producers in New Mexico affected by severe drought and wildfires. The agency expects to pay another $7 million this year—the most the state’s Livestock Forage Grazing Program has ever paid.

The Final Word

After a very dry winter the monsoon failed to deliver sufficient rainfall to substantially improve drought conditions in most of the Southwest. Many areas will enter the upcoming winter in need of widespread and heavy rain and snow.

La Niña conditions have returned, according to the National Oceanic and Atmospheric Administration (NOAA), and these events historically deliver dry conditions to the Southwest. Current forecasts issued by the International Research Institute for Climate and Society indicates greater than a 50 percent chance that the La Niña event will persist through February.

Although forecasts suggest this La Niña event will be weak, the strength of the event is not a good indication of the strength of the precipitation anomalies.

The November–January and December–February climate forecasts issued by the NOAA call for slightly increased chances for below-average precipitation in all of Arizona and New Mexico. The highest odds for below-average rain and snow are for the southern areas of both states, where La Niña impacts are often strongest.

Although the monsoon has ended, tropical storms in the eastern Pacific Ocean can cause heavy precipitation during October in the Southwest.

Thanks to La Niña, the Upper Colorado River Basin has a good chance of receiving average snowfall in mid-winter. However, a repeat of last winter’s record-breaking snowfall is unlikely.