Abstract
ECMWF forecasts of African easterly waves and tropical cyclogenesis in the Atlantic basin are investigated during 2020-2024, with a focus on Integrated Forecasting System (IFS) upgrades and the new Artificial Intelligence Forecasting System (AIFS). Ensemble-based probabilistic forecasts, valid at the time a tropical storm was named, exhibited high variability, with sensitivity to the maximum wind speed threshold. In many cases, the probabilities rose sharply when the lead time was reduced from 72 h to 48 h. The average probabilities have generally increased in each year, including when the IFS ensemble grid spacing was reduced from 18 km to 9 km in 2023. Across 18 developing tropical cyclones in 2024, the probabilities based on the AI-based ensemble system (“AIFS-CRPS”) often exceeded IFS probabilities for 84-120 h lead times, especially for stronger waves. In contrast, the AIFS-CRPS probabilities were lower for 36-48 h lead times, especially for the weakest waves. The average AIFS-CRPS ensemble mean position forecast error was often lower than that of the AIFS-Single, IFS deterministic, and IFS ensemble mean forecasts. The wave locations in the control (deterministic) IFS forecasts exhibited a greater southward bias than the single AIFS forecasts (“AIFS-Single”), whereas both models exhibited a slow bias in longitude. Mean absolute errors in AIFS-Single were mostly smaller for 3-5 day forecasts of 850-500 hPa wave-relative environmental vorticity and specific humidity, with lower biases in these fields. Overall, the AIFS-CRPS ensemble and AIFS-Single forecasts serve as a useful complement to the IFS.