Seasonal rainfall forecasts for drought situations in West Africa

Reliable seasonal rainfall forecasts are essential for improved early warning of large-scale droughts in West Africa but remain a major challenge for national meteorological services in the region. This study presents a statistical post-processing approach for improved probabilistic forecasting of seasonal rainfall amounts for the West Africa region. The novel approach relies on a circulation pattern approach to incorporate seasonal and interannual dynamics of West African monsoon processes, such as the Saharan Heat Low or the Tropical Easterly Jet, in combination with a simple logistic regression to predict rainfall amounts. The approach was tested in a reanalysis mode (1960 to 2010) for several climatic regions in West Africa using ERA5 in parallel to a regional station-based precipitation dataset and state-of-the-art global precipitation products such as CHIRPS. In addition, the statistical approach was applied to a hindcast period (1981 – 2023) for the peak monsoon period in West Africa and compared with the raw precipitation forecasts of ECMWF's SEAS5 and the real-time forecasts of the national weather services in West Africa subjectively produced as part of the West African Regional Climate Outlook Forum (WARCOF, 1998-2023). The study shows that the circulation pattern model outperforms both WARCOF and the raw rainfall forecasts of SEAS5. While WARCOF and SEAS5 show some forecasting skill for above and below normal conditions, both models show common limitations often observed in seasonal forecasting, such as lack of sharpness and a strong over-forecasting of near-normal conditions due to a risk aversion of the WARCOF experts. The circulation pattern-based approach provides much more accurate precipitation forecasts with greater reliability. Furthermore, a theoretical assessment of the economical value shows that the circulation pattern approach provides positive economic values for a wide range of potential users making it more suitable for forecasting rainfall amounts in drought situations in this region compared to SEAS5 and WARCOF. This study therefore provides a basic statistical post-processing approach for producing more accurate operational seasonal rainfall forecasts in the long-term for this challenging region.