The CEMS-Flood long-range forecasts comprise two model systems; seasonal (SEAS) for both EFAS and GloFAS and sub-seasonal to seasonal (S2S) (EFAS only) outlooks. The two systems use different meteorological forcing in their model chain and are issued with different frequency. The SEAS forecast is issued at monthly where as the S2S is issued. Up until EFAS v4.7, this was twice weekly, on Mondays and Thursdays. From EFAS v4.8, this is every day.
The forecasts provide hydrological outlooks which show the likelihood of high or low flows within the coming eight weeks in a region (aggregation of river basins). They are created by comparing the forecast with CEMS-Flood low and high flow thresholds, each averaged over the defined regions. The schematic below illustrates how the CEMS-Flood seasonal and sub-seasonal (EFAS only) outlooks are produced.
Schematic view of steps in producing CEMS-Flood seasonal and sub-seasonal (EFAS only) forecasts