Arizona Monsoon Thunderstorms.. Facts

A monsoon ... in ARIZONA? Surely you jest!?
Yes, indeed! Contrary to the denials of Arizona old-timers, we here in the Southwest do have a "monsoon."
How can Arizona, a desert, have a monsoon like India?
Actually one of the reasons that India has its more famous monsoon is largely due to the huge Rajasthan Desert in western India. But more fundamentally a 'Monsoon' is linked more to a wind shift rather than precipitation. In fact, the name "monsoon" is derived from the Arabic word "mausim" which means "season" or "wind-shift". Again, for India, during the winter dry period, the airflow comes from high pressure to the north—the dry Himalayas and Siberia. For the summer, the desert of western India heats up and low pressure forms. This causes air to swirl in from the west, the south and the east—all oceans! The result? HEAVY RAIN!

The Arizona Monsoon is a well-defined meteorological event (technically called a meteorological 'singularity') that occurs during the summer throughout the southwest portion of North America. During the winter time, the primary wind flow in Arizona is from the west or northwest—from California and Nevada. As we move into the summer, the winds shift to a southerly or southeasterly direction. Moisture streams northward from the Pacific Ocean and the Gulf of Mexico. This shift produces a radical change in moisture conditions statewide.

Such a change, together with daytime heating, is the key to the Arizona monsoon. This wind shift is the result of two meteorological changes:

The movement northward from winter to summer of the huge upper air subtropical high pressure cells, specifically the so-called Bermuda High (H).
In addition, the intense heating of the desert creates rising air and surface low pressure (called a thermal low) in the Mohave (L).
These two features combine to create strong southerly flow over Arizona. The southerly winds push moisture north-ward from Mexico. The exact source region for the moisture of the Arizona monsoon is unknown. Researchers have proposed the Gulf of Mexico and/or the Gulf of California as the source regions but conclusive evidence has so far been elusive.This has lead to the creation of large data-collecting efforts and research programs such as SWAMP, the Southwest Area Monsoon Project .

By the way, the term "monsoons" as in "when the monsoons arrive ..." is a meteorological no-no. There is no such beast. The word should be used in the same manner that "summer" is used. Consequently, the proper terminology is "monsoon thunderstorms" not "monsoons."

Monsoon thunderstorms are convective in nature. By that, we mean that the thunderstorms are powered by intense surface heating. In addition, strong moisture influx into Arizona is also required. The operational criterion for the onset of "monsoon" conditions used in Arizona is "prolonged (3 consecutive days or more) period of dew points averaging 55°F" or higher." There is nothing magical, however, about 55°F. It originally was linked to the total amount of water in the atmosphere above the weather station (a precipitable water amount of 1", a quantity thought to be necessary for convective thunderstorm activity). In general, for Phoenix, the temperature limits for the production of monsoon thunderstorms are 100° to 108°F with the optimum temperature being about 105°F. Temperatures needed to produce Tucson's thunderstorms are somewhat lower.

The Arizona monsoonal circulation does not produce thunderstorms every day during the months of July-September but rather occurs in a pattern that has "Bursts" and "Breaks". According to climatologist Andrew Carleton:

"Burst": a movement into Arizona of a weak trough in the upper level westerlies (normally during summer these winds are far north of this location) which spreads upper level cold air into the region. In lower levels, during a "burst", there is strong surface heating and strong southerly or southeasterly transport of moisture into Arizona. This creates intense atmospheric destabilization and leads to strong widespread thunderstorm outbreaks.

"Break": an enhanced ridging of the upper level Bermuda and North Pacific subtropical high pressure systems which acts to stabilize the atmosphere and thereby cutoff widespread thunderstorm activity. Our own meteorologists suggest that a break usually occurs when the subtropical ridge re-develops over NW Mexico and drier air spreads into Arizona.

During the monsoon, or summer thunderstorm season, Arizona experiences more severe weather than many other states. On rare occasion, a severe storm may spawn a tornado. More often, high winds, dust and severe downpours resulting in flash floods are common monsoon occurrences.

Prior to 2008 the Phoenix area monsoon was considered to have started when there were three consecutive days when the dew point averaged 55 degrees or higher. In 2008 the National Weather Service decided to take the guesswork out of monsoon start and end dates. After all, monsoon is a season, and most people should not be concerned with whether or not a particular dust storm was defined as monsoon storm or not. Beginning in 2008, June 15 was established as the first day of monsoon, and September 30 will be the last day. Now we can be more concerned with monsoon safety and less concerned with definitions.

More About Phoenix Monsoon

Meteorologists still track and report dewpoints and study monsoon weather patterns. Here are some technical monsoon facts for our area. The facts relate to dewpoint and the meteorological definition of monsoon and not the date on the calendar.
The average starting date of the monsoon in Phoenix is July 7.
The average ending date of the monsoon is September 13.
The earliest start date for the monsoon was June 16, 1925.
The latest start date for the monsoon was July 25, 1987.
The average date of the first break in the monsoon is August 16.
The average total number of monsoon days (where a monsoon day is considered one with an average dewpoint of 55 degrees or higher) is 56.
The greatest number of monsoon days was 99, recorded in 1984.
The fewest number of monsoon days was 27, recorded in 1962.
The greatest number of consecutive monsoon days was 72, from June 25 through September 4, 1984. This was also the greatest number of consecutive days with dew points of 60 degrees or higher.
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 monsoon occurred in 1924 with only 0.35 inches.