…The Arizona Monsoon…

Correct definition of Monsoon: Any wind that reverses its direction seasonally.

Wrong definition of Monsoon: Thunderstorms that occur in Arizona during the summer are called monsoons.

The best example of a monsoon on earth occurs over the Indian sub continent.  During the months of April through October...a southwest wind brings heavy rains to this region, while a dry northeast wind is prevalent during the remainder of the year. 

In North America...a similar situation occurs over much of Mexico.   For example, in Acapulco, rainfall averages 51.8 inches during the months of June through October, while only 3.3 inches falls during the remainder of the year.

In the United States, Arizona and New Mexico are located on the northern fringe of the Mexican monsoon.   For most of the year, winds aloft over the southwest U.S. are west to northwest.   During the summer, winds turn to a more south to southeast direction, importing moisture from the Pacific Ocean, the Gulf of California, and the Gulf of Mexico.

As this moisture moves into the southwest...a combination of orographic uplift (air being forced to rise by the mountains), daytime heating from the sun, and weak upper level disturbances moving across the region causes thunderstorms to develop across the region. 

On a typical day during the Arizona monsoon...thunderstorms develop first in the early afternoon over the higher mountains and the Mogollon Rim.   Rain cooled air from these thunderstorms, known as outflow, moves down from the high country and into the deserts.   Acting like a small scale cold front, this outflow causes the hot and moist desert air to rise, producing thunderstorms.   One of the most favorable situations for a large scale thunderstorm outbreak in Phoenix is when outflow boundaries approach from the Mogollon Rim and southeast Arizona simultaneously.  Over the higher deserts, storms generally occur during the mid and late afternoon, while activity is most prevalent over the lower deserts during the late afternoon and evening. On most days, thunderstorm activity ends altogether by around Midnight or 1:00 AM.

As these thunderstorms decay, microbursts, producing severe wind gusts are quite common.  Severe thunderstorm wind gusts are defined as those that equal or exceed 50 knots (57 mph).

During the years 1996 through 1999 damage from severe thunderstorm weather events in Maricopa county Arizona totaled more than $225 million in damage. On August 14 1996, a wind gust of 115 mph was recorded at the Deer Valley airport, and damage exceeded $160 million.   Sometimes moisture associated with hurricanes and tropical storms in the eastern Pacific can get caught up in the monsoon flow and affect Arizona.   When this occurs, continuous heavy rains can persist for 24 to 48 hours or longer.   The best example of this was the Labor Day storm of 1970.   The remains of tropical Norma produced severe flash flooding resulting in 23 deaths in central Arizona.

In Phoenix, the monsoon is considered to have started when we have three consecutive days when the dew point averages 55 degrees or higher.  The 55 degree threshold should be viewed as a guideline for the beginning of the monsoon...and not a hard and fast rule.   The average start date of the monsoon in Phoenix is July 7, while the average ending date is September 13.

In Phoenix...normal rainfall during July, August, and September is 2.65 inches. The wettest monsoon occurred in 1984 when we had 9.38 inches of rain.   The driest was in 1924 with only 0.35.

In Arizona, the highest rainfall amounts during the monsoon occur in the mountains, and in the southeast.   The driest areas are along the Colorado River valley in the far west.  One of the wettest locations in Arizona during July, August, and September is Greer in the White Mountains, where rainfall averages 11.46 inches.  By contrast, one of the driest is Yuma, in the far southwest, where the average is only 1.21.