July 2011 Vol. 1 Issue 2

 

PDF Version of July 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 extensive 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 the latest understanding of the monsoon, and subsequent issues will focus on current conditions.

Monsoon Summary: June 15 - July 4

Tropical Storm Arlene kicked off the monsoon season over the July 4 weekend in many parts of Arizona, but dry conditions continue to reign in New Mexico (Figure 1). Arlene, the first tropical storm of the season, formed in the Gulf of Mexico on June 29 and a few days later pushed moisture up the Gulf of California and through the Colorado River Valley. Humid conditions spread east and westward. Dew point temperatures—a measure of the moisture in the air—rose above 60 degrees Fahrenheit as far east as Tucson, high enough to favor convective storms.

While Arlene provided the moisture, a high-pressure system that had been building in the Southwest helped push the winds in from the east. As the moist air flowed northeast from the low deserts in southwest Arizona, topography helped initiate convective storms, while relatively strong winds steered the storms back to the west toward the moisture source. This scenario generally favors long-lived and drenching storms that can persist through the night.

Gulf surges that affect the Southwest occur this early in the season about every other year, and this one provided much needed precipitation for the state’s parched landscape. In many areas, this rainfall snapped a dry stretch that persisted for nearly three months.

New Mexico, on the other hand, did not benefit from the gulf surge. The dry conditions in part reflect the suppressed monsoon activity in the eastern portion of the Mexican monsoon region. For several weeks, dew points have been below values that favor storm generation in this area, and even though winds aloft have been blowing from the east into New Mexico, dry conditions in eastern Mexico have meant less moisture to tap. Rain will be more common in New Mexico once the monsoon ramps up in Chihuahua and western Sonora.

The start of the monsoon season began near the average onset period, which has been in first week of July, for southern Arizona and a bit earlier for southwest Arizona and northern areas of the state that have received rainfall. The onset in southern New Mexico, on the other hand, will begin later than the average, which has typically been before July 3. The pattern of rainfall throughout the region has not been typical. Generally, rains first begin in southeast Arizona and southern New Mexico. Plenty of time remains, though, for those areas yet to receive precipitation.

Monsoon Conditions

Monsoon precipitation has been insufficient to improve drought conditions in Arizona and New Mexico, according to the July 7 U.S. Drought Monitor. Exceptional and extreme drought conditions cover about 18 and 79 percent of the two states, respectively (Figure 2), which is similar to one month ago. Exceptional drought is defined as droughts that occur, on average, once in every 50 years, while extreme droughts occur once in every 20 years on average.

On June 25, before monsoon rains began in earnest, moderate to extreme drought covered most rangelands and croplands in the Southwest (Figure 3). Southwest Arizona has received more than 200 percent of its June 24–July 4 average precipitation (Figure 1); as much as 3 inches of rain fell in this region between June 30 and July 6.

Most of southern New Mexico and southeast Arizona, where drought conditions are most severe, received less than 50 percent of average precipitation between June 21 and July 4 (Figure 1).

Parts of the region saw several days of rain between June 15 and July 1. Less than 0.1 inches of rain fell in eastern New Mexico on June 28 and light rain fell in southeast Arizona on June 30.

The dry winter set the stage for the worst fire season on record in Arizona and New Mexico, where more than 930,000 and 968,000 acres have burned since the  year began, respectively. The most acres burned in Arizona and New Mexico prior to 2011 was about 762,000 and 552,000, respectively. In areas with severe fire burns, monsoon rains threaten to unearth loose soils in landslides and mudflows.

A monsoon storm on July 5 generated a dust storm that stretched nearly 100 miles wide and a mile high and raced across Phoenix at about 60 mph.

On July 6th, more precipiation fell in nine minutes in Douglass, AZ than in the previous nine months. Prior to the storm, only 0.27 inches of precipitation had fallen since Cctober 1.

The Final Word

The monsoon is in place and storms began in southern Arizona around the average monsoon start date in the first week of July; activity in New Mexico is lagging behind.

While copious rains have fallen in some places, early season activity does not ensure continuous rain throughout the season. In 2009 the monsoon began like gangbusters in southeast Arizona and New Mexico but fizzled a few weeks later. Monsoon rainfall in most of both states was less than 75 percent of average.

The July precipitation forecast issued June 16 by the Climate Prediction Center called for enhanced odds of below-average rain, which generally has held up.

Forecasts for the entire monsoon season generally have not been better than a forecast of equal chances for above-, below-, and near-average precipitation.

Forecasts call for above-average tropical storm activity in the Atlantic Ocean but below-average activity in the east Pacific Ocean.

The expectation is that monsoon rainfall likely will improve drought by at least one category; in areas with exceptional and extreme drought, monsoon precipitation likely will be insufficient to erase drought conditions.

June–September rainfall in southern Arizona and New Mexico need to exceed 125 percent of average to end drought, according to the National Climatic Data Center; however, there is less than a 10 and 5 percent chance this will occur in southern New Mexico and Arizona, respectively.