Abstract Despite decades of research, the Madden‐Julian Oscillation (MJO) remains challenging for global prediction systems, partly because its representation is sensitive to convection‐environment interactions. We evaluate a new prognostic convective closure in NOAA’s Unified Forecast System (UFS) that incorporates large‐scale moisture‐flux convergence (MFC) as a source alongside buoyancy. By allowing convection to respond directly to developing convergence anomalies, the scheme increases sensitivity to large‐scale moistening and produces more coherent MJO preconditioning. Using the UFS Seasonal Forecast System and composite diagnostics across nine MJO events, we show that the mixed closure generates a stronger and more realistic buildup of MFC over the central and eastern Pacific days before peak convection. This enhanced preconditioning, which is weaker in the traditional closure, is accompanied by improved MFC‐precipitation coherence and a more organized large‐scale MJO structure. These results illustrate how convective‐closure formulation can influence the large‐scale feedbacks critical for MJO representation.