The North American monsoon (NAM) is unique compared to other monsoon systems, because its northward expansion may depend on warm current in spring moving up the Mexican coast, and its extension into the desert southwest of the United States may depend on sea surface temperatures (SSTs) in the northern Gulf of California (GOC) exceeding about 29ëC. Processes warming SSTs in the northern GOC may be complex. As a result of these processes, as well as other factors governing the general atmospheric circulation in the monsoon region, certain monsoon seasons are more extreme than others in the desert southwest, and the onset of the summer rains exhibits certain variability. Strong monsoon convective events are associated with intense rainfall and damaging flash floods leading to monsoon-driven storms such as the one that struck Las Vegas, Nevada, on July 8, 1999, triggering a 50- to 100-year flood event and killing two persons. We performed a series of numerical experiments, using the non-hydrostatic NCAR/Penn State Mesoscale Model Version 5 (MM5) to simulate the monsoon onset in AZ in 1999. MM5 modeling results show a rapid increase in precipitable water and convective available potential energy (CAPE) and vanishing of convective inhibition index (CINE) over the northern GOC when SSTs increase from 29ëC to 30ëC. We will examine model atmospheric soundings around the GOC, to improve our understanding of two observed phenomena: 1. The absence of relatively heavy rainfall in Arizona prior to the northern GOC SSTs reaching 29.5ëC, and 2. the modifying of the marine boundary layer. The SSTs change may remove or weaken the inversion cap over the northern GOC, and deepen the boundary layer, allowing it to act as an important moisture source during gulf surge events and southerly flow conditions. Our modeling experiments attempt to understand the role of the northern GOC as a factor that governs the timing, intensity and extent of the heavy rainfall in Arizona.