Abstract Mesoscale convective systems (MCSs) contribute significantly to summer precipitation in the tropics and midlatitude. Although soil moisture (SM) effects on convection are globally recognized, its specific role on mature MCSs in East China remains unclear. Using convection‐permitting simulations spanning 22 summers, we find that convective cores within mature MCSs preferentially develop on the drier side of strong SM gradients (∼200 km). This is evidenced by a 2.5‐fold increase in core occurrences downstream of the steepest 10% of SM gradients versus a near‐uniform surface. SM gradients shape sensible heat flux gradients via evapotranspiration, while upstream pre‐storm rain‐producing clouds suppress surface available energy. These processes favor MCSs through enhancing near‐surface temperature gradients which strengthen moisture convergence and zonal wind shear. Our results highlight the critical role of SM gradients in favoring MCS propagation in East China. As climate change intensifies SM heterogeneity, improved land‐surface representation offers potential for advancing rainfall prediction and projection.

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