Monsoon Onset Dates on the Tohono O'odham Nation

The Tohono O'odham Nation (TON) in southwestern Arizona receives half of its annual rainfall during July through September. This rainy season is associated with the North American Monsoon, which is a regional atmospheric circulation that develops over southwestern North America. Determining a local onset date of the monsoon is important for meteorological and climatological studies. A method is developed using precipitation and dew-point analyses from the North American Regional Reanalysis dataset for a 24 year period. The annual onset date is determined based upon meeting or surpassing minimum areal precipitation and dew-point threshold criteria for three consecutive days. Onset dates for the TON determined with this method show agreement with regional onset dates determined by other objective methods as well as with local and regional onset dates determined using rain gage data. Using the method, historic monsoon season onset dates for the TON were determined and a composite monsoon climatology dataset was developed for use in future analyses.