Regional Differences: The North American Monsoon System
Adapted from: Michael Crimmins. 2006. Arizona and the North American Monsoon System. University of Arizona College of Agriculture and Life Science. Publication No. az1417.
How Does the Monsoon Affect Arizona?
The onset of the monsoon season has a different meaning to different sectors of the Arizona economy. In general, the monsoon brings much needed rainfall to the state but can inflict property damage and cause risk to human lives through thunderstorm winds, flash flooding, hail. and lightning. Maricopa County reported more than $225 million in property damage associated with monsoon thunderstorms for the period of 1996 through 1999 (National Weather Service-Phoenix 2006). The National Weather Service, emergency managers, and flood control districts work together to monitor monsoon thunderstorm activity and minimize risks to lives and property.
Electrical Power Delivery
The delivery of electric power through aboveground transmission lines is especially vulnerable to damage by monsoon thunderstorms. The combination of high wind events and intense cloud-to-ground lightning activity can cause transmission lines to fail. One particularly bad summer thunderstorm event in 1996 downed electric power lines across the Phoenix metropolitan region, knocking out power to over 250,000 customers (Haro and Green 1996). Some customers were without power for over a week.
Monsoon precipitation can, in rare cases, provide boosts to water storage in reservoirs when thunderstorms are widespread through a watershed and produce large amounts of runoff. Usually thunderstorms are localized and produce runoff that either quickly infiltrates or evaporates. High afternoon temperatures and full sunshine can quickly cause the previous day’s rainfall to reenter the atmosphere through evaporation or plant transpiration. Due to these factors, monsoon precipitation typically does little to alleviate long-term drought conditions or improve groundwater or reservoir water levels.
The arrival of the monsoon often impacts agricultural crop production in a negative way. Precipitation and high relative humidity values create wet conditions that are favorable for the development of insect pests and plant diseases (Olsen and Silvertooth 2001). The high relative humidity values associated with the monsoon can also cause heat stress in plants by reducing the ability of plant leaves to cool themselves through transpiration. The normal features of a thunderstorm (high winds, flooding, hail) also inflict damage directly to crops.
Much of the rangeland in Arizona is populated with grasses that respond to monsoon season precipitation. Summer is the growing season for these native perennial grasses. They rely on monsoon precipitation to support growth and the accumulation of biomass that can be eaten by wildlife and livestock (Cable 1975). A poor monsoon with below-normal precipitation can have significant impacts on the condition of rangelands across Arizona.
The arrival of significant monsoon rains can also mean a substantial decrease in wildfire activity across the state. The average number of wildfires decreases rapidly in early July, which coincides with the arrival of the monsoon (Mohrle et al. 2003). Precipitation, higher dew points, and lower temperatures can inhibit the growth of new fires and limit the spread of actively burning fires.
The North American monsoon system is an important circulation feature for Arizona due to its control of summer precipitation amounts across the state. Thunderstorm activity during the summer monsoon season can contribute to over half the annual total precipitation over the southeastern and higher elevation areas of Arizona. The northern and low desert portions of the state see little monsoon thunderstorm activity except during large-scale thunderstorm outbreaks. The strongest and most active portion of the North American monsoon system is located in Mexico; Arizona is located on the northern periphery of this region. Complex interactions between circulation features and topography interact to guide moisture from this core region of activity into Arizona. The position and strength of these circulation features varies over time (during the summer season and from year to year), leading to high variability in precipitation amounts received across Arizona during the summer monsoon season.
Adapted for eXtension.org by Sabrina Kleinman, University of Arizona
Cable, D.R. 1975. Influence of precipitation on perennial grass production in the semidesert Southwest. Ecology. 56: 981-986.
Haro, J.A. and G.D. Green, 1996. The Southern Arizona Severe Weather Outbreak of 14 August 1996: An Initial Assessment. Western Region Technical Attachment No. 96-27, National Weather Service.
Mohrle, C.R., B.L. Hall, and T.J. Brown, 2003: The Southwest Monsoon and the Relation to Fire Occurrence. Fifth Symposium on Fire and Forest Meteorology, Orlando, Fla., American Meteorological Society.
National Weather Service-Phoenix, cited 2006. The Arizona Monsoon. Available online: http://www.wrh.noaa.gov/psr/general/monsoon/index.php.
Olsen, M. and J. C. Silvertooth, 2001: Diseases and Production Problems of Cotton in Arizona. University of Arizona Cooperative Extension Report AZ4215, 20 pp.
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