This paper uses rawinsondes and pilot balloon data from the 2004 North American Monsoon (NAM) Experiment, as well as satellite-based products and reanalysis datasets that span 1982 to 2018, to analyze the mixing mechanisms responsible for the temporal and spatial variations of the Gulf of California (GoC) boundary layer during the NAM onset. We show that the regional diurnal cycle is strongly affected by low-level convergence and divergence associated with local breeze regimes and by the presence and intensity of a thermal inversion over the gulf. Earlier starting monsoons have less moisture available for precipitation than those starting later in the calendar year. Therefore, early onset monsoons have less rainfall during their first month, which is a result that is in contrast with previous studies that have analyzed the timing of the NAM but only reported seasonal precipitation totals. The GoC boundary layer height at the time of monsoon onset, found to be controlled by the gulf’s surface temperature, has a significant impact on the precipitation over Sonora, Sinaloa, and southern Arizona. After the erosion of the thermal inversion over the GoC that coincides with the NAM onset, wind shear produced by the region’s unique geographic and topographic features is the largest source of turbulence for the mixing of the boundary layer. Hence, our results suggest that a numerical model used to forecast or analyze NAM precipitation must have enough spatial resolution to adequately reproduce the effects that the GoC’s features have on its complex diurnal circulation systems.