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Date: 6/24/2013

Title: North American Monsoon

Not to be confused with its counterpart in India, the North American Monsoon is a vitally important source of rain during the summer season over many areas of the Desert Southwest, the central and southern Rockies and portions of western southern Plains.
The North American Monsoon is the main reason for the very busy pattern of late day and evening thunderstorm activity that usually develops in late June, July and August across the southwestern United States and into the Rockies. Sometimes, the impacts of the North American Monsoon can extend all the way into September. The intensity of the North American Monsoon can make or break it for cattle producers in some portions of the west. The very heavy and sometimes frequent rains from the North American Monsoon can bring the type of rainfall amounts that can refresh rangeland conditions in a matter of weeks.
What drives the North American Monsoon? Simply put it is the heat of summer and the complex terrain of Mexico and the western United States. During the development of the North American Monsoon, there is a shift in wind patterns in early summer which occurs as Mexico and the southwest U.S. become very warm under intense solar heating during June and July. The deserts get hot and dry and this causes a natural area of low pressure to form over Mexico and the Desert Southwest. Air naturally moves from high pressure to low pressure. High pressure is located over the eastern Pacific to the west of Mexico and the Gulf of Mexico to the east. Therefore, air over the oceans (high pressure) begins to move to the land (low pressure) during the heat of summer. The prevailing winds start to flow from moist ocean areas into dry land areas. This is illustrated in the graphic below.




With higher humidity air over the landmasses, the combination of daytime heating along with the natural lift of air along the slopes of the mountainous terrain makes it very easy for thunderstorms to form in the moisture rich air mass that is overhead during the North American Monsoon. The end result is heavy rain producing thunderstorms.
The monsoon begins in late May to early June in southern Mexico and quickly spreads along the western slopes of the Sierra Madre Occidental, reaching Arizona and New Mexico in early July. If upper level wind conditions are favorable, the monsoon will spread north into Colorado, Wyoming, Utah and the Four Corners region. Sometimes the monsoon moisture will enhance rains as far north as Montana and South Dakota and as far east as western Nebraska and western Kansas.
Tropical storm or hurricane activity moving in from the Pacific from the west or from the Gulf of Mexico to the east can also enhance the North American Monsoon by feeding even more moisture into Mexico.
Monsoons play a vital role in managing wildfire threat by providing moisture at higher elevations. With the wildfire season already going strong in New Mexico and Colorado this season, a strong North American Monsoon season will be critical to aid fire-fighting.
Flash flooding is a serious danger during the monsoon season. Dry washes can become raging rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away. In fact, of the biggest and deadliest flash floods in the west happen during the North American Monsoon. The Big Thompson Flood in Colorado on July 31, 1976, the Cheyenne, Wyo., flood of Aug. 1, 1985 and the Fort Collins, Colo., flood of July 28, 1997 are good examples.
In addition to the very moist air that feeds the thunderstorms during the monsoon season, the jet stream winds aloft are very weak. Weak winds aloft mean that the thunderstorms will move very slowly or even stall over an area, leading to possible flash flooding.
All indications suggest that the North American Monsoon is going to be kicking into gear over the next two weeks. Badly needed rainfall will be possible in the Desert Southwest, New Mexico and into portions of Colorado during the first two weeks of July.