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Extratropical transition can extend the threat of tropical cyclones into the midlatitudes and modify it through expansion of rainfall and wind fields. Despite the scientific and socioeconomic interest, the seasonal forecast of extratropical transition has received little attention. The GFDL High-Resolution Forecast-Oriented Low Ocean Resolution (FLOR) model (HiFLOR) shows skill in seasonal forecasts of tropical cyclone frequency as well as major hurricanes. A July-initialized 12-member ensemble retrospective seasonal forecast experiment with HiFLOR in the North Atlantic is conducted, representing one of the very first attempts to predict the extratropical transition activity months in advance. HiFLOR exhibits retrospective skill in seasonal forecasts of basin-wide and regional ET activity relative to best track and reanalysis data. In contrast, the skill of HiFLOR in predictions of non-ET activity is limited. Future work targeted at improved predictions of non-ET storms provides a path for enhanced TC activity forecasting.

Plain Language Summary Extratropical transition (ET) is the process that tropical cyclones evolve from warm-core symmetric systems to cold-core asymmetric systems. Tropical cyclones undergoing transition can extend the threat of storms to midlatitudes by severe fresh flooding associated with enhanced rainfall (e.g., Hurricane Agnes, 1972) and storm surge associated with storm reintensification (e.g., Hurricane Sandy, 2012). Seasonal forecasts of ET activity have the potential to provide guide for storm preparedness and risk management. However, there have been few studies on this topic. This study provides the first attempts to predict ET activity months in advance in the North Atlantic using a global climate model. The model exhibits good skill in predicting basin-wide and regional ET storm frequency. In contrast, limited skill in predictions of non-ET storm frequency points to the need for improvement in future.