The land-sea thermal contrast (LSTC) that is present at the initial stage of the North American Monsoon is analyzed as a principal driving mechanism for monsoon onset interannual variability. The vertically integrated moisture flux convergence (MFC) averaged over the core monsoon during June 16–30, derived from North American Regional Reanalysis daily fields for the period 1979–2006, is proposed as an index for initial monsoon intensity. We quantify the LSTC associated to the monsoon and propose a dynamic connection between the thermal contrast and the initial monsoon intensity. It consists of a directly proportional relation between the LSTC, the surface pressure gradient along the Gulf of California, and the ensuing low level (below 850 mb) moisture transport (from the southern Gulf of California and eastern tropical Pacific) and precipitation in the core region.