The purpose of this study was to assess trends in the intensity of the North American monsoon anticyclone over multiple decades from 1948 to 2010 during July and August, with a focus on the Lower Colorado River Basin (LCRB). The methodology included a 500 hPa geopotential-height regionalization of the monsoon-anticyclone domain (i.e. a large portion of the western United States and northern Mexico), a typing of 500 hPa circulation patterns over the LCRB, and an examination of multi-decadal trends as well as inter-epochal differences in geopotential heights and frequencies of synoptic types. Three regions (i.e. Northwest, Northeast, and South) were revealed that differed based on inter-annual variations in 500 hPa geopotential heights. The Northwest and South regions had significant increases in geopotential heights from 1948–1978 to 1980–2010. The synoptic types reflected the location of the monsoon anticyclone over the LCRB. The monsoon anticyclone intensified primarily over the northwestern region, which includes the LCRB, since the mid- to late 1970s. The anticyclone has thus been expanding over the LCRB. The anticyclone has been in a northern position, specifically a north-central position, over the basin more frequently over the past 30 years; conversely, the anticyclone has been in southern and eastern positions over the basin less frequently.