Abstract The climatological western North Pacific summer monsoon onset, a so-called convection jump, occurs around the 41st pentad, corresponding to an abrupt northeastward extension of strong convection. This study investigates the process of convection jump from a local perspective. Composite analyses are performed based on the onset dates that are identified in individual years. The results show that the convective inhibition (CIN) decreases dramatically around the onset dates, while the convective available potential energy (CAPE) reaches its maximum long before the onset, suggesting that the CIN, rather than CAPE, plays a dominant role in triggering convection. Further analysis indicates that the reduction of CIN is induced by the increased low-level relative humidity, which is the result of enhanced water vapor convergence. The moisture transportation is primarily contributed by the wind transfer from easterlies to southeasterlies or southerlies along the southern boundary of the convection jump region, in accordance with the monsoon trough establishment. The present observational results may be used to evaluate climate models in simulating the stepwise evolution of summer monsoon. Significance Statement The convection jump refers to rapid convection extension to the northeastern part of the Philippines Sea in late July and corresponds to the deepening of the monsoon trough and northward shift of the western North Pacific subtropical high. Therefore, the convection jump has important impacts on the East Asian summer monsoon. The crucial role of transient atmospheric adjustment in triggering the convection jump has been shown in previous studies. However, the reason for the temporal preference of convection jump remains unknown. This study further specifies the process of atmospheric adjustments around the convection jump from a local perspective and reveals that the transition from the easterlies to southeasterlies or southerlies decreases convective inhibition and thus favors the convection jump. The present results can be used to diagnose the ability of climate models to simulate the stepwise evolution of Asian–Pacific summer monsoon.