This forecast is developped in the frame of the SeSaM SCO project (CLS, IRD, MOi, CNES) and is based on the NEMO-Sarg1.0 ensemble configuration provided by IRD/LEGOS (Jouanno et al., 2021), forced by NEMO4.2 ocean and SEAS5 atmospheric data. Initial conditions are obtained from S3-OLCI detections of sargassum distribution, provided by CLS.