<p>This study aims to analyze the synoptic patterns conducive to extreme rainfall (ER) over the South China Coast (SCC) and the low-level thermodynamic structure over the sea. Using observational and reanalysis data, a clustering analysis of the 700 hPa synoptic conditions is conducted during the pre-summer rainy seasons of South China from 1988 to 2017. The conditions are classified into three patterns: Type-L (Low-pressure type), Type-I (Intermediate type), and Type-H (High-pressure type). The circulation characteristics and diurnal variations associated with ER in each pattern are examined. Type-L is characterized by a low-level southwest airflow from the South China Sea (SCS), which strengthens coastal convergence, providing abundant moisture and favorable thermodynamic conditions. Therefore, Type-L is associated with more ER events than the other patterns. In Type-I, the SCC lies between a shallow trough and a ridge, influenced by southerly airflow. Type-H is dominated by a high-pressure system over the eastern SCS, where moisture conditions are weaker than in the first two patterns, leading to fewer ER events. During ER events, a significant warm belt below 850 hPa forms on the lee side of the Indochina Peninsula due to descending motion. In all synoptic patterns, a profound low-level warm advection occurs from the southwest. However, moisture transfer paths notably differ. In Type-L, low-level moisture primarily originates from the western SCS, driven by strong southwesterly winds in the boundary layer. In contrast, moisture in Type-I and Type-H is sourced from the central and eastern SCS, respectively. The diurnal variation of low-level winds is a key factor in controlling the diurnal cycles of moisture transport and rainfall. The enhancement of ER at night is related to the land-sea thermal contrast over the SCC, coupled with an increase in moisture and meridional winds. Among the three patterns, Type-I is primarily driven by local thermal convection, which plays an important role in ER.</p>