September 2011 Vol. 1 Issue 4

PDF version of September 2011 Monsoon Tracker

Preface

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 - September 1

The summer has not been a nonsoon, but it hasn’t been 2006 either, the last year when copious rains doused most of Arizona and New Mexico. Across the region, rainfall generally has been below 75 percent of average since July 1 (Figure 1)—the image excludes several days of monsoon activity between June 15–30, but theses events do not substantially alter the rainfall pattern. August precipitation also was slightly below average in most of the region, with the most robust storm activity hovering over southeast Arizona and southwest New Mexico. In July and August rain was heavier in the mountains, which is common during the monsoon season. A lack of consistently strong winds aloft and a reduced temperature difference between the surface and mid-altitudes in the atmosphere—which has created a more stable environment—have limited storms from forming and propagating over the desert valleys.

Despite underwhelming rainfall totals in August, the past 30 days have not been mundane. August will be remembered for record heat across the entire region, continued bone-dry conditions in eastern New Mexico, and dust storms that blew through central Arizona later than usual. The high temperatures and low moisture can be partially blamed on the position of the subtropical high, or monsoon ridge. The ridge has been persistently centered over Oklahoma and Texas for most of the monsoon season, which is too far east to continuously funnel moisture into southeast New Mexico and Southwest Arizona. The position of the ridge is both a cause and consequence of the exceptionally dry conditions in Texas and southeast New Mexico. As a result, hot and dry conditions in eastern New Mexico have continued in August—precipitation has been less than 50 percent of average—and average temperatures have been 6–10 degrees Fahrenheit above average.

In recent weeks dew points—a measure of the moisture in the air—dipped below levels that favor the formation of monsoon storms. In Phoenix, for example, 12 of the 14 days between August 19 and September 2 had dew points below 55 degrees F. Low moisture has had a feedback effect, amplifying temperatures because clouds have been less present and rain storms, which cool the environment, have been few and far between. As a result, many areas of Arizona and New Mexico recorded there  warmest August on record, which extends for 117 years.

Temperatures at night also have set record highs in many areas. The warm conditions and meager rain have enabled two massive dust storms, or haboobs, to barrel through central Arizona in August. Although haboobs are common during the monsoon, they typically occur earlier in the season, before showers cake the topsoil with a crust that guards against strong winds. The record dry winter and lackluster monsoon undoubtedly primed this area for dust storms.

Monsoon Conditions

Drought conditions remain exceptional in most of southeastern New Mexico, and all of New Mexico is experiencing moderate or more severe drought conditions; moderate drought or a more severe drought category in Arizona covers about 70 percent of the state (Figure 2).
Monsoon rainfall totals in many parts of the Southwest were higher in August than July, which helped drive modest improvements in drought conditions in parts of both states. A large portion of southeast Arizona and southwest and northwest New Mexico improved one drought category in the last month, from exceptional to extreme drought.
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.

In southeast Arizona and southwest New Mexico, rainfall more than 200 and 150 percent of average, respectively, is needed to end drought conditions. There is less than a 3 percent chance that this will happen, based on historical data. In southeastern New Mexico, more than 200 percent of average rainfall is needed to curb drought conditions. This also has less than a 3 percent chance of occurring.
Hot weather reigned in August. The average low temperature in Phoenix was 87.5 degrees F, making it the warmest average low on record for the month. In Flagstaff, Winslow, Prescott, and Tucson, the average August temperatures were the warmest or second warmest on record.
Extreme events have cost more than $35 billion in the U.S. since the beginning of the year. The ongoing drought and fires in Arizona, New Mexico, and several other western states have cost more than $5 billion (estimated before the outbreak of destructive fires in Texas in early September).
The The combined water storage in Elephant Butte and Caballo reservoirs on the Rio Grande in New Mexico is 202,000 acre-feet, or 9.6 percent of the 2.23 million acre-foot capacity. Last year at this time, the water volume was nearly twice as high.

The Final Word

The one-month lead forecast for September issued by the NOAA-Climate Prediction Center 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 chances that a La Niña event will be present for a second consecutive winter are increasing. Forecasts issued by the International Research Institute for Climate and Society indicate a 44 percent chance that a La Niña event will develop during the November–January period, an increase from 26 percent forecasted last month. It is important to note that the probability of neutral conditions continuing this winter still has the highest odds at 54 percent.
Back-to-back La Niña events could accentuate the acute drought impacts already gripping the region—in the historical record, the second year in backto-back events has tended to be drier than the first.
The position of the subtropical high has been persistently centered over Oklahoma and Texas, too far east to favor widespread monsoon storms. It has helped limit the flow of moisture into eastern New Mexico from the south and blocked surges originating in the Gulf of California. The low moisture has also increased temperatures, compounding the severity of drought.