Towards dynamical seasonal forecast of extratropical transition in the North Atlantic

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.