NOTE: this post has been updated. The previous code was based on the conversion of the GRIB files into NetCDF, which introduces unfortunately some issues.
Among the data products of the Copernicus Climate Change (C3S) available through the Climate Data Store, there is a collection of seasonal forecasts, from the 13th November consisting of five different models (ECMWF, UK Met Office, Meteo-France, DWD and CMCC).