The North American Monsoon

Overview for North American Monsoon

The North American Monsoon (sometimes called the Mexican Monsoon or the Arizona Monsoon or the Southwest United States Monsoon) is experienced as a dramatic increase in rainfall from an extremely dry late spring season to a rainy summer season. The North American Monsoon region includes northwestern Mexico and parts of the US desert southwest. While the most notable change in our weather is the increase in moisture and precipitation during the monsoon season, it should be pointed out the meteorological definition of a Monsoon is simply a seasonal shift in wind direction. Monsoonal shifts in winds around the world are often associated with changes in precipitation, but they need not be.

On average, the climate of Earth is characterized by a belt of high 500 mb heights at around 30° North latitude. This semi-permanent ring of high 500 mb heights is called the subtropical high. Lower 500 mb heights are found both to the north and to the south of the subtropical high region. The wind flow at 500 mb is parallel to the height contour pattern, with lower heights to the left of the wind direction. This means the 500 mb winds are generally west toward east north of the subtropical high and east toward west south of the subtropical high. (See Figure showing average 500 mb height pattern and winds.) For most of the year, Tucson is north of the subtropical high and the 500 mb winds over Tucson are generally west to east. At lower levels in the atmosphere, air south of the subtropical high is humid tropical air, while the air near the subtropical high is much drier.

The North American Monsoon is associated with the northward movement of the position of the subtropical high that occurs from June through September. As the subtropical high moves to the north of Tucson, the 500 mb winds shift direction to become east to west. This is an indication of the monsoonal wind shift that happens over the southwestern US and northern Mexico. (see Figure showing average 500 mb height pattern and winds during the monsoon season.) During the spring months of the year, the subtropical high pressure cell is south of Arizona, and Arizona experiences generally westerly air flow with generally dry air masses. During the summer monsoon season, the center of the 500 mb subtropical high moves north of Arizona. This places Arizona in generally easterly air flow with much more humid, tropical air masses. The specific center of high heights that forms in this part of the world during the summer season is often called the monsoonal high. Rainfall during the monsoon season is not continuous. It varies considerably, depending on a variety of factors. There are usually distinct "burst" periods of heavy rain during the monsoon, and "break" periods with little or no rain. Monsoon precipitation, however, accounts for a substantial portion of annual precipitation in northwest Mexico and the Southwest U.S. Most of these areas receive over half their annual precipitation during the monsoon season.

500 mb Pattern and Air Flow during the Monsoon Season in Tucson

As mentioned above, the typical 500 mb air flow over the southwestern United States is generally westerly (west to east). This is a consequence of the fact that for most of the year, the average air temperature between the ground and 500 mb gets colder as one moves toward the north, which means lower 500 mb heights are found toward the north and the corresponding 500 mb winds are west to east (parallel to the height contours with lower heights to the left of the wind direction). One indicator of the southwest monsoon is to have higher 500 mb heights form to the north of southern Arizona. This results in a wind shift at 500 mb, with 500 mb winds that have an easterly component, i.e., winds from east toward west. This can be understood by applying the rule for 500 mb wind direction. With higher 500mb heights to the north of Tucson, the 500 mb wind direction will be generally east to west.

The meteorological definition of a Monsoon is simply a seasonal shift in wind direction. Often a monsoon season is also characterized by a shift in precipitation (as it is in the southwestern United States and India), but it need not be. The term monsoon is often misused, even by many TV weather reporters. It is incorrect to call a summer thunderstorm a "monsoon" or to say it is "monsooning" outside during a summer thunderstorm. There is a monsoonal shift in winds that happens every year in the southwestern United States and northwestern Mexico though. This shift in wind patterns is most easily seen by looking at the seasonal shift in wind direction that occurs at the 500 mb pressure level. Below are links to higher resolution monthly average 500 mb height maps that have been annotated to show the monsoonal shift in wind direction that takes place during the months of July and August over the southwestern United States. In the figures below the 500 mb height contours are shown every 10 meters, instead of the more common every 60 meters. This is done to better show the wind direction at 500 mb in Tucson. The bold 'H' indicates where the highest 500 mb heights are located in the vicinity of the southwestern United States. The bold arrow indicates the 500 mb wind direction in Tucson based on the average height pattern shown. Notice that the average wind direction is southwest (from the southwest) during the month of June. This is prior to the monsoon season. During the monsoon months of July and August, the 500 mb high moves to the north and east of Tucson and the average wind direction is from the east and southeast. In September, the high center moves back to the south of Tucson, the wind direction shifts back to southwest, which indicates the end of the monsoon season.
June Average 500 mb heights and Tucson wind direction
July Average 500 mb heights and Tucson wind direction
August Average 500 mb heights and Tucson wind direction
September Average 500 mb heights and Tucson wind direction

Please also read over the following link, North American Monsoon article by the NWS. You should understand graphics 2 and 3, which show the monsoonal wind shift at the 500 mb height levels (Graphic 1 shows wind patterns just above the ground surface, which we are not covering in this course). Graphic 3 shows one favorable position for the "monsoon high" in the sense that there are southeast winds at 500 mb over Tucson. As long as there is an easterly component to the 500 mb wind direction, Tucson will generally be favored for thunderstorms. Basically if the high center is north of Tucson, there will be an easerly component to the 500 mb wind direction. The chance for thunderstorms also depends on whether or not there is sufficient water vapor in the atmosphere as discussed in the next section.

In reality, the local 500 mb height pattern changes from day to day and a monsoonal height and wind pattern comes and goes throughout the months of June through September. It is common for there to be periods of several days to a week with favorable conditions for thunderstorms followed by several days to a week of unfavorable conditions. The officially defined monsoon season runs from June 15 through September 15. When the center of highest heights is south of Tucson, rain generally does not fall in Tucson. Worse yet, when the high center is directly overhead, not only is it generally rain-free, but also quite hot as the core of warmest air is right over us. The best chances for summer rains in Tucson happen when the 500 mb high center is north of our area. An example of a 500 mb height pattern that is unfavorable for summertime thunderstorms in Tucson is shown in the map for 0600 GMT, June 23. On that map, there is a high center located to the southwest of Tucson, and the 500 mb windflow over Tucson is from the northwest, which is not favorable for clouds and rain. An example 500 mb height pattern which is more favorable for thunderstorms in Tucson is shown in this example of a 500 mb pattern favorable for thunderstorms in Tucson. The timestamp at the bottom of the map 00Z, Thursday, August 9, 2012 corresponds to a local time in Tucson of 5 PM Wednesday, August 8, 2012. The approximate center of highest heights is indicated with an "H" along with several arrows showing the wind trajectory at various places on the map. One thing to point out is that the 500 mb height over Tucson is above 5940 meters. This is well above the average height of just over 5880 meters for Tucson during the month of August. Thus, we would expect warmer than average temperatures in the area, which was the case as a record high of 107°F was recorded. With regard to the possibility of summer thunderstorms, notice that the high center is north of Tucson and the 500 mb wind flow is northeasterly (from the northeast). This is a favorable wind direction for monsoon season thunderstorms in Tucson as there is an easterly component to the wind direction. In fact thunderstorms did form in the mountains to the east and northeast of Tucson in the early afternoon and by 5 PM had moved into the Tucson area. Remember that the movement of individual thunderstorms is often in the direction of the 500 mb winds. An interesting weather product provided by the department of Atmospheric Sciences is a movie loop of the 500 mb height pattern over the previous 24 hours. In addition to the 500 mb height pattern, satellite-derived water vapor imagery is superimposed on the images. Without going into details, the water vapor imagery distinguishes areas of high water vapor content and clouds. This allows you to visualize the movement of these features with respect to the 500 mb pattern. Bright white cloud masses are generally steered around by the 500 mb winds.

Increase in Water Vapor

The monsoon season happens every year in the southwestern United States. While a monsoon is defined by the seasonal change in winds, monsoons are often associated with changes in humidity and precipitation. The change in water vapor content during the North American Monsoon is obvious when looking at the changes in precipitable water over a year, for example, Tucson daily average precipitable water for 2011.

As you may know if you are from Tucson or as you may learn if you follow local weather during the monsoon season, accurately predicting whether or not Tucson will get thunderstorms on a given day is very difficult. The department of Atmospheric Sciences runs a local weather model, which makes forecasts for a portion of the southwest US and northwest Mexico. If you are interested in an alternative weather forecast for Tucson, check out Regional Weather Forecast Models and Products for southeast Arizona from ATMO WRF Model.

Weather forecasting during the Monsoon Season

Wind Barb Symbols on Weather Maps

The wind barb symbol is a common way to indicate the wind speed and wind direction at points on weather maps. The wind barb consists of a line segment starting at the location (point on a map) where the wind information is provided and points in the direction from which the wind is coming with shorter line segments and flags attached to the end of the direction line segment to specify wind speed. For a wind from the north the barb sticks straight up, from the east the barb points right, from the south the barb points straight down, and from the west the barb points left. Wind speed is indicated by the line segments and flags attached to the end of the barb. There are long and short line segments. Each long line segment represents 10 knots of wind (there can be up to 4 long line segments). A short line segments represents an additional 5 knots of wind. If the wind speed is less than 5 knots, then an open circle is drawn on top of the location where the wind is indicated. By the way a knot is a nautical mile per hour (1 knot = 1.15 miles per hour). You should look over the links below.

(Figure D Reading wind barb symbols) ; (Legend for wind speed)

500 mb forecast maps with wind barbs from the University of Oklahoma

ECMWF forecast of 500 mb heights and winds (Courtesy of University of Oklahoma Weather Lab)