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 degrees N, 150-170 degrees 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 degrees N, 130-150 degrees 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. (C) 2011 Elsevier B.V. All rights reserved.