Two ensemble forecasting systems are currently operational at ECMWF: (1) the EPS (Ensemble Prediction System) for medium-range weather forecasting and (2) the seasonal forecasting (Seasonal Forecast System, so-called: S5). The EPS produces weather forecasts out to 15 days, whereas seasonal forecasting (S5) produces forecasts out to 7 months.
The two systems have different physical bases. Medium-range weather forecasting (EPS) is essentially mostly driven by the atmospheric initial state and a complex assimilation system provides the best initial states.Seasonal forecasting (S54), on the other hand, is justified by the long predictability of the oceanic circulation (of the order of several months) and by the fact that the variability in tropical SSTs has a significant global impact on the atmospheric circulation. The oceanic circulation is a major source of predictability in the seasonal scale, the ECMWF seasonal forecasting system is based on coupled ocean-atmosphere integrations. Seasonal forecasting is also an initial value problem, but with much of the information contained in the initial state of the ocean.
It becomes obvious that between EPS and S5 there exists a certain gap. The main goal of the extended range forecast is to fill the gap between these systems. Twice a week ECMWF produces forecasts for the time range 15 to 46 days. The time range 15 to 46 days is probably still short enough that the atmosphere retains some memory of its initial state and it may be long enough that the ocean variability has an impact on the atmospheric circulation. Therefore, the extended range forecast has been built as a continuation of the medium-range VAREPS (VAriable Resolution EPS)
Furthermore, an important source of predictability over Europe in the 15-46 day range is believed to originate from the Madden Julian Oscillation (MJO). The MJO is a 40-50 day tropical oscillation. Several works suggest that the ocean-atmosphere coupling has a significant impact upon the speed of propagation of an MJO event in the Indian Ocean and western North Pacific. The use of a coupled system helps to capture some aspects of the MJO variability.