Predictive Skill and Predictability of North Atlantic Tropical Cyclogenesis in Different Synoptic Flow Regimes

Practical predictability of tropical cyclogenesis over the North Atlantic is evaluated in different synoptic flow regimes using the NCEP Global Ensemble Forecasting System (GEFS) reforecasts with forecast lead time up to two weeks. Synoptic flow regimes are represented by tropical cyclogenesis pathways defined in a previous study based on the low-level baroclinicity and upper-level forcing of the genesis environmental state, including non-baroclinic, low-level baroclinic, trough-induced, weak tropical transition (TT) and strong TT pathways. It is found that the strong TT and weak TT pathways have lower predictability than the other pathways, linked to the lower predictability of vertical wind shear and mid-level humidity in the genesis vicinity of a developing TT storm. Further analysis suggests that stronger extratropical influences may lead to lower genesis predictability. It is also shown that the regional and seasonal variations of the genesis predictive skill in the GEFS can be largely explained by the relative frequency of occurrence of each pathway and the predictability differences among pathways. The impact of genesis pathways on predictability is discussed through a thought experiment using the concept of the genesis potential index.