Strong low-level southerlies lasting for several days are defined as the southerly monsoon surge (SMS), which plays an important role in the moisture transport and production of heavy rainfall over North China. Based on reanalysis and observation data, this study analyzes the evolution of SMS during a heavy rainfall episode in July 2025 and the impacts of both synoptic systems and thermodynamic forcing of terrains. The results show that the variation of 700-hPa southerlies was closely related to the development of west Pacific subtropical high (WPSH) and westerly trough, which were the dominant systems for the northward moisture transport over North China. The diabatic heating of Mt. Taihang led to a significant mountain-plain thermal contrast, which further enhanced the southerlies in planetary boundary layer (PBL), leading to the increment of 1–2 m s^-1 for southerly wind at 925 hPa than that at 700 hPa, which was beneficial for the enhancement of convergence in PBL. Furthermore, the southerly wind in PBL featured a significant diurnal variation due to the composite effects of Holton mechanism and Blackadar mechanism. Its nocturnal acceleration led to the enhancement of moisture transport over North China plain, which together with the mountain breeze effects of Mt. Yanshan enhanced the convergence and rainfall near the windward slope during nighttime.

extreme rainfall, summer monsoon, diurnal variation