The EFAS long-range forecasts comprise two model systems; seasonal (SEAS) and sub-seasonal to seasonal (S2S) 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 twice weekly, on Mondays and Thursdays.

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). The regions were defined by merging several river basins together, while respecting the underlying hydroclimatic boundaries. Moreover, the river basins were aggregated in order to capture the large-scale variability in the weather. The hydrological outlooks are created by comparing the forecasts with EFAS low and high flow thresholds, each averaged over the defined regions. The schematic below illustrates how the EFAS seasonal and sub-seasonal outlooks are produced.

EFAS seasonal and sub-seasonal forecasts
Schematic view of steps in producing EFAS seasonal and sub-seasonal forecasts

EFAS seasonal and sub-seasonal products

There are two outlook products produced for all regions at the start of each forecast, viewable under the ”Seasonal outlook” and "Sub-seasonal outlook" layers. The overview maps indicates the development of the water in the coming weeks.

  • Overview map: regional discharge anomaly over the forecast horizon, highlighting regions with highest or earliest probability of a high (>90th percentile) or low (<10th percentile) discharge anomaly.

Screenshot of EFAS overview map for seasonal and sub-seasonal outlooks
Screenshot of EFAS overview map for seasonal and sub-seasonal outlooks
 

  • Time series information: weekly averages of ensemble discharge forecast as box plots. The seasonal forecasts (left) are updated weekly with the latest LISFLOOD discharge simulation to help the forecaster with real-time validation. The sub-seasonal forecasts (right) are updated biweekly to give the user the latest forecast information.

EFAS seasonall (left) and sub-seasonal (right) time series for a selected region
EFAS seasonal (left) and sub-seasonal (right) time series for a selected region
 

To know more

Arnal, L., Cloke, H. L., Stephens, E., Wetterhall, F., Prudhomme, C., Neumann, J., Krzeminski, B., and Pappenberger, F., 2018: Skilful seasonal forecasts of streamflow over Europe? Hydrol. Earth Syst. Sci., 22, 2057-2072, https://doi.org/10.5194/hess-22-2057-2018

Wetterhall, F. and Di Giuseppe, F.: The benefit of seamless forecasts for hydrological predictions over Europe, Hydrol. Earth Syst. Sci., 22, 3409–3420, https://hess.copernicus.org/articles/22/3409/2018/