Insights from Ex-Typhoon Halong (2025)—An Arctic Cyclone of Tropical Origin

Abstract An Arctic cyclone, ex-Typhoon Halong, produced strong winds and devastating flooding in southwestern Alaska during 11–12 October 2025. This study examines the evolution of Halong after its transition into an extratropical cyclone through the analysis of ERA5 reanalysis and simulations by the Weather Research and Forecasting (WRF) Model. It is found that positive sea surface temperature (SST) anomalies over the western North Pacific preconditioned ex-Halong for intensification by increasing water vapor content and reducing static stability. Quasigeostrophic lifting associated with a subsequent interaction with another extratropical cyclone led to the rapid deepening of ex-Halong. This case demonstrates that tropical cyclones can transition into extratropical systems that are intensified by anomalously warm ocean waters, exacerbating impacts in high latitudes. Further analyses indicate that an increasing fraction of Alaskan cyclones has originated in tropical latitudes (south of 30°N) in recent decades. In particular, the frequency of Arctic cyclones of tropical origin increased fourfold in August and threefold in September during 1980–2025 compared with 1940–79. Significance Statement This study investigates the mechanisms behind the severe impacts of ex-Typhoon Halong on Alaska in October 2025. Our analysis demonstrates that unusually warm sea surface temperatures preconditioned the reintensification of the storm in the extratropics. This intensification was further facilitated by the storm’s interaction with an existing extratropical weather system, leading to rapid deepening. Beyond this specific event, the frequency of Arctic cyclones of tropical origin has increased fourfold during late summer since 1980. These findings suggest that as ocean temperatures continue to rise, tropical-origin storms may increasingly pose a major risk to high-latitude communities and environments.