Orographic Effects on Precipitation From Hurricane Helene

Abstract Hurricane Helene was deadly and destructive, with orographically driven precipitation in the Appalachians contributing to historic rainfall and flooding. Here, we quantify the orographic effects on precipitation from Hurricane Helene using a novel set of global, cloud‐resolving simulations with and without the Appalachian Mountains using the Simple Cloud‐Resolving E3SM Atmosphere Model (SCREAM). We found that the Appalachians redistributed tropical cyclone precipitation, producing over 300 mm more upslope rainfall driven by enhanced vertical motion, while suppressing leeward rainfall through reduced condensation and accretion. The pathways to precipitation formation are similar in both simulations: cloud condensate primarily forms from vapor to liquid water condensation, and accretion is the main rain formation mechanism. On the windward side, orography enhances these mechanisms through a seeder‐feeder‐like process, with higher precipitation efficiency indicating that the mountains produce a more efficient storm with respect to converting available moisture into surface rainfall.