Seasonal Analysis of the 2011–2017 North American Monsoon near its Northwest Boundary

The seasonal extent of the North American Monsoon (NAM) is highly variable and potentially sensitive to future climate change. Our objective was to determine how regional monsoonal patterns influence mountain precipitation near the NAM northwest boundary. Among the data we analyzed, a unique opportunity was provided by hourly observations collected on the Sheep Range (2300 m asl), in the Mojave Desert of southern Nevada, during 2011-2017. Long-term 800-m Parameter-elevation Relationships on Independent Slopes Model (PRISM) precipitation time series showed that the site is representative of mountain areas in the NAM northwest region. Based on in situ observations, we divided the water year into three seasons: cool (1 October through 31 March), early warm (1 April through last day with dewpoint <9.4 degrees C), and late warm (first day with dewpoint >= 9.4 degrees C through 30 September). Dewpoint temperature differed by about 8 degrees C between early warm season (mean of -6.3 degrees C) and late warm season (mean of 2.3 degrees C). According to ANCOVA model results, increasing hourly dewpoint associated with afternoon thunderstorms in the late warm season had the greatest relationship with hourly precipitation (F-value = 237.8, p-value < 0.01). Except for 2016, more precipitation fell at our study site during the late than the early warm season. Late warm season precipitation contributed the most (43-56%) to total water-year precipitation during the 2012-2015 extended drought. Southwestern USA regional composites of vertically integrated water vapor transport (IVT) suggested that water vapor in the cool and early warm season originated from the Pacific Ocean to the west, while a transition to a NAM-like pattern of northward IVT coincided with the late warm season.