The North American Monsoon (NAM) brings heavy precipitation during the summer months to portions of Arizona, New Mexico, Colorado, Utah, Nevada, and California, northwest Mexico. In many regions of the southwestern United States, the NAM is the primary source of water. From one year to the next the NAM varies both spatially and temporally, which makes it difficult to predict. Studying the NAM is important because if a relationship between winter snow cover and summer precipitation can be developed, then it may be possible to better predict the onset, intensity, and duration of the NAM.

Onset, length of monsoon season, and number of days within the monsoon season with precipitation were obtained from Ellis and Hawkins (2001). Winter and spring snow cover totals for North America at a 1° x 1° resolution were obtained from the Rutgers University Global Snow Lab database. Daily gridded meteorological data were taken from the National Surface Water Modeling Group at the University of Washington and includes daily precipitation totals for stations that are available at a resolution of 1/8°. Data from 283 grid cells in Arizona and New Mexico were used to analyze the relationship between snow cover and monsoonal precipitation.

Based on the results of the analysis (1967–2001) an average onset (July 4th), ending date (September 18th), and season length (77 days) were defined. Winter snow cover is more strongly correlated with the onset and length of the NAM than spring snow cover. Winter snow cover is positively correlated with date of onset, and negatively correlated with the length of season and the number of precipitation days. Preliminary results also suggest that there is a moderate negative correlation between average monsoon precipitation and snow cover. Monsoon rainfall is strongly correlated with the date of monsoon onset and termination. Not surprisingly, the amount of precipitation that falls during the monsoon is directly related to the how early the monsoon season starts, how long it lasts, and how many precipitation days occur during the monsoon season. Monsoon precipitation tends to be greater during years that have an early onset, longer than normal season with an above average number of precipitation days. Investigations of the predictability of monsoonal precipitation may provide a basis for forecasting water deficits in urban areas.