Summertime Precipitation in the Lake Mead Watershed

The North American Monsoon (NAM) is the main driver of summertime climate and variability in the American southwest. While much work has been done on the NAM, most focuses on the region defined as Tier I in the seminal North American Monsoon Experiment (NAME), a region that stretches from central-western Mexico to southern Arizona and New Mexico. This work presents a climatological characterization of summertime precipitation, defined as July, August, and September (JAS), in the Lake Mead watershed, located in the NAME Tier II region, spanning from 1981-2016. Spatial and temporal variability of JAS rainfall characteristics are examined using PRISM 4km daily precipitation data. The importance of the number of wet days (24hr rainfall ≥ 1 mm) and extreme rainfall events (95^th percentile of wet days) to the total JAS precipitation is also examined. A strong west-to-east gradient exists with respect to drivers of interannual variability of JAS precipitation with extreme events playing a larger role in the west and total number of wet days in the east. An investigation into the dynamical drivers of extreme rainfall events, as defined above, indicate that many extreme events in Tier II are caused by upper-level disturbances from the west (cyclonic Rossby wave breaking) in the form of cut-off and closed lows. This is in contrast to the NAME Tier I region, where extreme events are frequently associated with easterly upper-level disturbances (inverted troughs).