Monsoon Season

A monsoon is a seasonal change in atmospheric circulation and precipitation associated with the differential heating of land and and ocean. This differential heating, where the land is warmer than the water, results in large scale changes to the wind patterns allowing moisture-laden air to flow from the ocean sources to the land.  We typically associate monsoons with southern Asia and subtropical locations.  However, the North American Monsoon is an important mechanism bringing rain to the southwestern U.S. For many locations the monsoon accounts for 50 to 60 percent of the average annual precipitation.

The Southwest Arizona Monsoon Project (SWAMP) in 1990 and 1993 established the fact that a true monsoon, characterized by large-scale wind and rainfall shifts in the summer, develops over much of Mexico and the intermountain region of the U.S.  In 2004 the North American Monsoon Experiment (NAME) conducted in northwest Mexico and the southwest U.S sought to better describe the monsoon in North America, and increase our ability to predict it on a daily, weekly and seasonal basis. NAME showed that the weather pattern in the southwestern U.S.during the summer is not only a true monsoon, but it also affects the weather over a large portion of North America.

This map depicts the general weather pattern for the
North American Monsoon.The thermal low sets up
over the Desert Southwest,while the subtropical high
moves into the Southern Plains. The resulting wind
pattern draws moisture from sources in the Gulf of California,
the Sierra Madre mountains, and the Gulf of Mexico (green arrows).
Source: NOAA National Weather Service

The North American Monsoon (NAM) is associated with an area of high pressure, the subtropical ridge, that moves northward during the summer months and a thermal low (a trough of low pressure which develops from intense surface heating) over the Mexican Plateau and the U.S. Desert Southwest. The NAM usually occurs in five general phases, although this can vary from year to year. In general the monsoon season extends from June 15th to September 30th. The Ramp-Up phase usually occurs from mid-June through early July. The onset of the NAM occurs from late June through mid-July.  At this time winds over the southwestern U.S. shift into the east and southeast drawing moisture in at both the surface and aloft. The National Weather Service officially marks the start of the monsoon when there have been three consecutive days with the dew point above 54°F.  The peak of the NAM occurs from mid July through mid August.  The Late Monsoon phase occurs from mid August to mid September, and the monsoon Decay can occur from late August to late September.

While the monsoon brings beneficial rain to the normally parched southwestern U.S., it also produces a variety of weather hazards including intense heat, enhanced wildfire risk, flash flooding, severe thunderstorms with damaging winds and hail, lightning, and dust storms.

The North American Monsoon is a busy time for CoCoRaHS observers in Arizona, New Mexico, and southwestern Texas.  After weeks if not months of reporting zeroes they are finally able to measure something other than dust and bugs in the gauge. Check out the Daily Comments on the CoCoRaHS web site to get the observers' perspectives on the North American Monsoon.

You can learn much more about the North American Monsoon at two web sites in particular. The National Weather Service office in Tuscon, AZ has assembled an excellent series of web pages on the Monsoon in Southeastern Arizona and was the source for much of the information in this post.   There is also a web site dedicated to monsoon safety and preparedness, monsoonsafety.org.