Performance evaluation of real-time sub-to-seasonal (S2S) rainfall forecasts over West Africa of 2020 and 2021 monsoon seasons for operational use

Olaniyan, E. A., Woolnough, S. J. ORCID: https://orcid.org/0000-0003-0500-8514, Andrade, F. M. D. ORCID: https://orcid.org/0000-0001-6653-3916, Hirons, L. C. ORCID: https://orcid.org/0000-0002-1189-7576, Thompson, E. ORCID: https://orcid.org/0000-0003-4250-8075 and Lawal, K. A. ORCID: https://orcid.org/0000-0002-8198-8844 (2025) Performance evaluation of real-time sub-to-seasonal (S2S) rainfall forecasts over West Africa of 2020 and 2021 monsoon seasons for operational use. Atmosphere, 16 (9). 1072. ISSN 2073-4433 doi: 10.3390/atmos16091072

Abstract/Summary

Accurate sub-seasonal-to-seasonal (S2S) forecasts are critical for mitigating extreme weather impacts and supporting development in West Africa. This study evaluates real-time ECMWF S2S rainfall forecasts during the 2020–2021 West African monsoon (March–October) and uses corresponding hindcasts for comparison. We verify forecasts at 1–4 dekads lead against two satellite-based rainfall datasets (TAMSAT and GPM-IMERG) to cover observational uncertainty. The analysis focuses on spatio-temporal monsoon patterns over the Gulf of Guinea (GoG) and Sahel (SAH). The results show that ECMWF-S2S captures key monsoon features. The forecast skill is generally higher over the Sahel than the GoG, and peaks during the main monsoon period (July–August). Notably, forecasts achieve approximately 80% synchronization with observed rainfall-anomaly timing, indicating that roughly 4 out of 5 dekads have correctly predicted wet/dry phases. Probabilistic evaluation shows strong reliability. The debiased ranked probability skill score (RPSS) is high across thresholds, whereas the average ROC AUC (~0.68) indicates moderate discrimination. However, forecasts tend to under-predict very low rains in the GoG and very high rains in the Sahel. Using multiple datasets and robust metrics helps mitigate observational uncertainty. These results, for the first real-time S2S pilot over West Africa, demonstrate that ECMWF rainfall forecasts are skillful and actionable (especially up to 2–3 dekads ahead), providing confidence for early-warning and planning systems in the region.

Item Type	Article
URI	https://centaur.reading.ac.uk/id/eprint/124480
Identification Number/DOI	10.3390/atmos16091072
Refereed	Yes
Divisions	Science > School of Mathematical, Physical and Computational Sciences > NCAS
Publisher	MDPI
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