In a significant milestone, a new generation of sub-seasonal and seasonal (longer-range) river flow outlook products was implemented recently in the European and Global Flood Awareness Systems (EFAS & GloFAS) of the Copernicus Emergency Management Service (CEMS). ECMWF is the computational centre of CEMS, which is led by the EU's Joint Research Centre (JRC).
The new longer-range river outlook products provide improved consistency in the applied styles and methods across forecast horizons (i.e. sub-seasonal and seasonal) and geographical domains (i.e. European and global). Other novelties include better usability with a navigation option in the web map viewer to see maps of different lead times; the quantification and representation of forecast uncertainty on the map products; and the introduction of more anomaly categories, especially for the extreme tail of the distribution and near-normal conditions. The anomaly categories are all provided with associated uncertainty (see the example snapshot of one of the sub-seasonal forecasts in EFAS in the first figure).

Main novelties
The revamping of the products was done consistently with existing ECMWF products. For example, the sub-seasonal forecasts are now produced daily, consistent with ECMWF's sub-seasonal forecasts since Cycle 48r1 of the Integrated Forecasting System (IFS), introduced in June 2023. They are valid for the next six calendar weeks (Monday to Sunday). Similarly, seasonal products are produced monthly and are valid for the next seven calendar months, consistent with seasonal forecast products provided by the Copernicus Climate Change Service (C3S).
Real-time longer-range EFAS and GloFAS forecasts, based on river discharge averaged over calendar weeks and calendar months, are generated using ECMWF and C3S forecasts: sub-seasonal forecasts, enhanced by high-resolution medium-range forecasts in the first 15 days, and SEAS5 seasonal forecasts. These serve as input for the LISFLOOD distributed hydrological model developed by the JRC.
The reliability of any forecast anomaly product strongly depends on the relevance of the underlying reference model climatology. The new longer-range river flow outlook products have adopted the approach of using re-forecasts to generate a model climatology that changes both with seasons and with forecast lead times, similar to how it is done for ECMWF sub-seasonal and seasonal forecast anomalies.
The forecast anomaly and uncertainty information is derived by comparing the real-time 51-member ensemble to the underlying reference model climatology, represented by 100 equally likely bins, and by ranking the members in the climatological distribution (see the second figure for a schematic representation of the method). These ranks define how usual, unusual or extreme the weekly-averaged or monthly-averaged river flow values are at the time of year and at the specific forecast lead time. The expected forecast anomaly is then determined by the mean of the 51 rank values, while their standard deviation defines the uncertainty around the predicted anomaly.

Accessing the products
There are four new, freely accessible product layers introduced on the EFAS (https://european-flood.emergency.copernicus.eu/) and GloFAS (https://global-flood.emergency.copernicus.eu/) websites. The 'Sub-seasonal/Seasonal Outlook' maps highlight the expected forecast anomaly/uncertainty signal on simulated rivers, and the 'Sub-seasonal/Seasonal Outlook – Basins' maps show the spatial summary of the 'Outlook' layer by aggregating the signal onto larger scales at predefined basins. This is done as an arithmetic mean of the rank mean and rank standard deviation values at the river pixels in the basins, weighted by the square root of the catchment area, which gives more weight to larger rivers.
In addition, the 'Outlook' map also includes the reporting point pop-up products, which give detailed information on antecedent and forecast conditions and can be accessed by clicking on point markers.
Please go to the dedicated wiki page (https://confluence.ecmwf.int/display/CEMS/Sub-seasonal+and+seasonal+forecast+products) for detailed documentation on the EFAS and GloFAS sub-seasonal and seasonal products and the generation methodology.