The GloFAS long-range forecasting system has two flavours, the seasonal and the sub-seasonal outlooks. The two systems use different meteorological forcing in their model chain and are issued with different frequency. The seasonal forecast is issued monthly (by around the 8th for the 1st of each month), while the sub-seasonal is issued daily from 28 June 2023 and twice weekly, on Mondays and Thursdays, before that. The GloFAS seasonal products were revised in GloFAS v4.3 (implemented on 04 June 2025), while the sub-seasonal products were added as new in the same upgrade. A short description of the earlier system and products is available further below.
The GloFAS sub-seasonal and seasonal outlook layers shows the 'expected to happen' forecast anomaly and uncertainty. The forecast signal's temporal resolution is calendar monthly out to 7 months in the seasonal and calendar weekly out to 5-6 weeks (depending on the forecast run date) in the sub-seasonal. The forecast anomaly and uncertainty signals are based on the extremity level of each ensemble member, which is derived by comparing the real time forecast to the underlying model climatology (with 99 percentile values). The climatology is generated using reforecasts over a 20-year period, which provides range-dependent climate percentiles that change with the lead time.
Two web layers are available for both the sub-seasonal and seasonal systems on the GloFAS website. The first one is the river network map ('Seasonal Outlook' and 'Sub-seasonal Outlook'), which highlights the forecasts on all river pixels above 1000 km2 catchment area; and also includes reporting points with popup windows delivering extra information about the evolution of the antecedent, climate and forecast conditions. The second layer is the basin summary map ('Seasonal Outlook - Basins' and 'Sub-seasonal Outlook - Basins'), which shows the aggregated forecast signal over about 1000 basins. The two map products are provided for each lead time individually through a web functionality, which allows the users to navigate the maps and go backward and forward within the forecast horizon.
The schematic below illustrates how the GloFAS seasonal and sub-seasonal outlooks are produced. Further information on the details of the different steps generating the sub-seasonal and seasonal forecast signal and the related products is available at CEMS-flood sub-seasonal and seasonal forecast product generation methodology.
Schematic view of the steps in producing the GloFAS seasonal and sub-seasonal forecasts and products.