GloFAS meteorological forcing data sets, for the different forecast simulations, are coming from the latest ensemble of Numerical Weather Prediction (NWP) system of the ECMWF IFS, with different model configurations used for the different GloFAS lead times and systems. Below, the main use of the meteorological forecast data sets, used as forcing for the hydrological modes, is described.
GloFAS medium-range (15-day)
The medium-range ensemble forecasts (ENS) are used to force the GloFAS system out to 15-day lead time. ENS is the ensemble of possible forecast scenarios, produced by ECMWF, and consists of 51 members at ~9 km resolution up to 15 days, produced twice a day at 00 and 12 UTC. ENS is used to force the the medium-range GloFAS (from the 00 UTC forecast run).
In addition to the general ensemble use, there are aspects of the GloFAS system (in the so-called fillup part of the model initialisation) that uses specifically the ENS-CNTL, which is a special member of the ensemble, as it uses the most accurate estimate of the current initial conditions, while the other 50 ensemble members start from slightly perturbed conditions.
GloFAS sub-seasonal
The combination of the medium-range (ENS) and sub-seasonal (or extended-range; SUBS)) ensemble forecasts is used for the sub-seasonal GloFAS forecasts and for the GloFAS reforecasts. The sub-seasonal GloFAS forecasts and reforecasts use the same methodology to combine the high-resolution medium-range forecasts at 9 km resolution and the lower-resolution sub-seasonal-range meteorological forecasts at 36 km resolution.
The real-time high-resolution ENS is available twice a day at 00 and 12 UTC with 51 ensemble members, while SUBS is only available once a day at 00 UTC with 101 ensemble members. Therefore the real-time GloFAS sub-seasonal forecasts are produced once a day at 00 UTC with 51 ensemble members out to 46-day lead time, in a way that the high-resolution meteorological data is used in the first 15 days, while for the remaining period of 16 to 46 days, the first 51 members of the sub-seasonal forecasts are taken as meteorological forcing, both from the 00 UTC run of the same date.
The combination of the two higher- and lower-resolution forecasts is done in a simple manner, in which the 1st ensemble member of the medium-range forecast (in the first 15 days) will be followed by the 1st ensemble member from the sub-seasonal forecasts (in the 16-46-day range), and so on the same with all of the 51 members. Although, the combination of the two different meteorological forecasts at two different resolutions can create some local discontinuities at day15, especially at complex orographical areas, on average the GloFAS ensemble forecasts are not expected to suffer any major disturbance after day15 and rather just benefit from the added value of the higher resolution meteorology used in first 15 days of the forecasts.
The GloFAS reforecasts, on the other hand use the exact same configuration as the GloFAS sub-seasonal forecasts, but they are only produced about every 4 days at 00 UTC, following the generation schedule of the ECMWF reforecasts. These meteorological reforecasts are produced always on fixed days of the months (1, 5, 9, 13, 17, 21, 25 and 29, expect 29 Feb in leap years), for two flavours on higher- and lower-resolution, similar to the real-time forecast, but only with 11 ensemble members. The combination of the lower- and higher-resolution meteorological forecasts before and after 15 days is the same as for the real time forecasts, but this time only with 11 members, not 51.
GloFAS seasonal
The seasonal-range NWP forecasts of ECMWF, SEAS5 (SEAS5) is used to force the GloFAS seasonal forecasts and the GloFAS seasonal reforecasts. SEAS5 is the ensemble seasonal forecast of ECMWF (operational since 5 November 2017), and one of the seasonal forecast models of the Copernicus Climate Change Service. It consists of 51 members at ~36 km horizontal resolution, generated each month (from the 1st of the month) with 7 months lead time (extended to 13 months once a season). SEAS5 forecasts are available in reforecast-mode from 1981 to 2016 with 25 ensemble members, while from 2017 the usual 51 ensemble members are available, produced in real time.
GloFAS 30-day (legacy)
GloFAS also has a 30-day forecast simulation prodcued, which was the base for the operational real-time forecasts before the version 4.3 update in June 2025. This 30-day forecast is very similar to the sub-seasonal forecast configuration described above, in fact, it is identical to it in the first 15 days. However, it has a shorter lead time at 30 days only (instead of 46 days), while it combines the high-resolution meteorological forecasts (of the first 15 days) with 1-day old lower-resolution meteorological forecasts (for day 16 to 30). As the sub-seasonal forecasts are available considerably later than the 15-day medium-range ones (for the same 00 UTC run), the real-time generation of the GloFAS 30-day forecasts in the morning was only possible by using the 1-day old sub-seasonal meteorological forecasts (until GloFAS v4.2). Since GloFAS v4.3, the medium-range GloFAS web products are based only on the first 15 days, while the new sub-seasonal products are based on the new sub-seasonal forecast configuration with lead time of 46 days.
In addition, the users currently are provided continued access to these 30-day GloFAS forecasts in the CEMS Early Warning Data Store (EWDS).