Skill and Consistency of ECMWF Forecasts of Atlantic Tropical Cyclone Genesis

Abstract We evaluate the skill and jumpiness of the ECMWF medium-range ensemble (ENS) in predicting tropical cyclone genesis in the Atlantic basin. Focusing on the probabilistic performance of the ENS, we assess how far in advance the ENS can predict genesis, quantify the consistency (jumpiness) from run to run, and investigate what factors influence the skill and consistency. We find that first indications of genesis are picked up at least 7 days ahead in 50% of the observed cases, although strong signals often only appear less than 3 days before genesis. There are significant regional differences, with observed genesis events predicted 2–3 days earlier in the eastern Atlantic than in other areas. The genesis probabilities can be jumpy from run to run, and the jumpiest cases are in the more skillful regions (central and eastern Atlantic) and for situations where the initial signal for genesis appears at longer lead time. In the eastern Atlantic, there is a tendency for the ENS tracks to reach tropical storm strength earlier and further east than observed; this model bias can affect both skill and jumpiness of the genesis forecasts. Our results provide guidance to forecasters on how to use and interpret the ENS predictions. Areas for future work include the link between early intensification in the eastern Atlantic and African easterly wave activity, the relationship between skill and the TC development pathways, and the impact of systematic analysis differences between 0000 UTC and 1200 UTC on forecast intensity. Significance Statement Forecasting where and when tropical cyclones will appear increases the lead time at which decision-makers can begin to take preparatory mitigating action. Numerical weather prediction models can provide important guidance but sometimes are not consistent from one run to the next. We evaluate the skill and consistency of a state-of-the-art global model in predicting the formation of tropical cyclones up to 10 days ahead and provide guidance to forecasters on how to use and interpret the model predictions. We show that the formation of tropical cyclones can be predicted 2–3 days earlier in the eastern Atlantic than in the western Atlantic and identify some of the factors influencing both skill and consistency.