This study investigates the atmospheric conditions in the mid-North Pacific Ocean prior to the western North Pacific summer monsoon (WNPSM) onset by focusing on 3 years (2007, 2009, and 2010) with the CloudSat Cloud Profiling Radar (CPR) observation. Two pentads prior to the onset, the mid-latitude upper-tropospheric high potential vorticity (PV) expanded southwestward and intruded into the mid-troposphere of the region around Marcus Island (Marcus region, 20–30°N, 150–170°E). Due to the intrusion of PV, the convection developed over the Marcus region, and the Marcus convergence zone (MCZ) formed. One pentad later, the WNPSM onset occurred when the mid-tropospheric high-PV center and the convection arrived at the subtropical western North Pacific (SWNP, 15–25°N, 130–150°E). It is found that the high-PV expansion is in association with the deepening of the mid-oceanic trough, which signifies the upper-level forcing on the monsoon. It is also found that the cloud water content increased significantly since two pentads prior to the monsoon onset over the mid-North Pacific Ocean. The MCZ-induced moisture advection might provide favorable large-scale atmospheric conditions for changing monsoon-phase.