Conditional Weather Regimes and Moisture Pathways Associated With Summer Rainfall Over the Southeastern United States

ABSTRACT Summer rainfall over the southeastern United States (SEUS) is influenced by regional moisture transport, vertical motion, and large‐scale circulation variability, yet persistent wet periods can occur within different circulation configurations that are obscured by a mean wet composite. Here, we examine SEUS summer rainfall during 1979–2023 using a conditional weather‐regime framework in which persistent wet periods are first identified from 5‐day mean rainfall anomalies and the associated circulation states are subsequently classified using 850‐ and 200‐hPa geopotential height anomalies. Four conditional regimes (CRs) are identified, each characterized by distinct coupled lower‐ and upper‐tropospheric circulation anomalies. The composite western ridge of the North Atlantic Subtropical High (NASH) in CR A is located farther west, near 85° W, whereas CR B, CR C and CR D have closely clustered ridge locations near 80° W and 26°–27° N. Despite these similar western‐ridge locations, the regimes differ substantially in the surrounding NASH‐associated pressure field and moisture transport. In CR A and CR B, an anomalous ridge along the northern flank of the NASH is accompanied by stronger zonal moisture convergence, which provides the dominant contribution to net moisture convergence over the SEUS. In contrast, in CR C and CR D this ridge‐like anomaly is weaker or absent, zonal convergence is reduced, and the meridional contribution becomes relatively more important. All four regimes are also characterized by tropospheric ascent over the SEUS, indicating that the distinct moisture‐transport environments coincide with rainfall‐supporting vertical motion. Rossby wave source and wave activity flux diagnostics further indicate regime‐dependent upper‐level wave environments across the North Pacific and North America, with CR A exhibiting the clearest trans‐Pacific wave‐activity pathway. Overall, the results reveal distinct circulation–moisture pathways to persistent SEUS rainfall, underscoring that similar regional wet states can arise from fundamentally different large‐scale dynamical configurations.