Monsoon Summary

A gulf surge from Tropical Storm Arlene kicked off the monsoon rainfall in many parts of Arizona during the July 4 weekend,which was right around the average onset date for the southern part of the state. Copious rains soaked the parched southwestern landscape along the Colorado River Valley, up into Nevada and Colorado, and as far east as Tucson. However, storms did not form over New Mexico, hindering improvement in drought conditions in the state. After the July 4 weekend, dew points fell below 50 degrees Fahrenheit in southern Arizona for nearly one week and dry conditions returned to the Southwest—dew points above 50 degrees F usually favor the formation of thunderstorms. Between July 14 and 21, convective storms returned to Arizona and finally made their way into New Mexico.

In the past 30 days, since June 21, rains have totaled less than 3 inches in most of the Southwest, with many areas experiencing less than 1 inch (Figure 9a). These totals are below average for many areas, particularly New Mexico (Figures 9b–c). Southern New Mexico, which is experiencing exceptional drought—a drought that occurs about once in every 50 years—has been the driest. Monsoon moisture essentially has skipped this area in part because the eastern half of the monsoon region in Mexico has been drier than average, limiting the moisture available to flow into New Mexico. In addition, the position of the high-pressure monsoon ridge aloft has been slightly to the east, preventing water vapor from the south from moving into the state. Some areas have fared better than others in New Mexico since the beginning of July. Western counties along the Arizona-New Mexico border have seen the most consistent convective activity, and the southern mountain regions have also fared reasonably well. However, elsewhere the hit-or-miss character of the summer precipitation has resulted in fewer- than-normal opportunities for needed rainfall. Precipitation forecasts for August, issued by the NOAA-Climate Prediction Center (CPC), call for an equal chance of above-, below-, or near-average rainfall.