GloFAS Medium-range flood forecasts

How are GloFAS forecasts generated?

From GloFAS 3.1 (May 2021), GloFAS modelling chain is based on the Open Source LISFLOOD hydrological model, forced by an ensemble of meteorological forcings. ECMWF reanalysis and day-1 of ECMWF ENS control forecast are used to initialize the forecasts, followed by medium and extended range ensemble forecasts up to 30 days from the ECMWF Integrated Forecast System (IFS) ENS. ECMWF-ENS is the ensemble forecast of ECMWF and consists of 51 members at ~9 km resolution up to 15 days, increasing to 36 km from day 16 to 30. In GloFAS, the outputs of the 00:00 UTC IFS medium-range runs are used daily for day 1 to 15, and the latest available IFS extended range runs used for day 16 to 30. This page provides a detailed description of the meteorological forcings used in GloFAS.

The resulting simulated daily hydrological forecasts are compared with reference discharge runs obtained from the same modelling chain. This results in a set of probabilities of exceeding pre-defined flood thresholds of different severity.

Which products does GloFAS offer?

GloFAS offers a range of forecast products and associated information, accessible via different layer tabs of the GloFAS map viewer:

• Under the initial condition tab, maps of meteorological or land surface variable at the start of the forecast, or up to 3 days before (e.g. precipitation)
• Under the meteorological tab, maps of expected precipitation for the next 10 days
• Under the hydrological tab, maps of where a flood signal is expected over the GloFAS river network
• Under the flood risk tab, maps of expected flood inundation extent and associated risk category
• Under the evaluation tab, maps of hydrological model performance and forecast skill

All GloFAS products are updated daily. Detailed information on all GloFAS products can be found in the dedicated GloFAS wiki pages
The GloFAS User Guide provides hand-on information on how to explore the Map Viewer.

GloFAS forecasts reference runs and data

GloFAS hydrological reanalysis is a set of deterministic time series of hydrological variables, including river discharge, runoff water equivalent, snow depth water equivalent, soil wetness index (root zone). GloFAS hydrological reanalysis is produced using atmospheric forcing as close as possible to observations, and the same modelling chain as the operational GloFAS to avoid the introduction of systematic biases. GloFAS hydrological reanalysis is used as initial conditions of GloFAS and GloFAS Seasonal reforecasts, to calculate discharge thresholds for GloFAS, and to conduct GloFAS and GloFAS Seasonal forecast skill assessments.

GloFAS reforecasts aare forecast simulations performed for a set of past dates, based on a configuration as close as possible to GloFAS operational setting, initialised using GloFAS hydrological reanalysis. GloFAS reforecasts can be used to analyse GloFAS forecast skill systematically over a long period.

GloFAS forecasts discharge thresholds are a set of maps of discharge magnitudes corresponding various return periods, from 1.5 years to 500 years. They are generated by fitting an extreme value distribution to the annual maxima series extracted from GloFAS discharge timeseries of the hydrological reanalysis (the data and a short description of the methodology can be found here, section GloFAS Flood Thresholds). GloFAS discharge thresholds are used to generate flood awareness forecast maps.

GloFAS data

GloFAS data is a gridded dataset at 0.05° horizontal resolution from GloFAS 4.0 (July 2023), at 0.1° horizontal resolution for all the previous versions. It includes:

• 51-member ensemble real-time daily forecasts for the next 30 days.
• Ensemble daily discharge reforecasts: the frequency (daily/twice weekly Monday and Thursday) and ensemble size depends on the reforecasts period (GloFAS reforecasts).
• GloFAS hydrological reanalysis based on the latest ECMWF reanalysis
• A set of deterministic flood thresholds return period discharge levels (GloFAS discharge thresholds).

Reanalysis, forecast, reforecast, and reanalysis dataset include the following key hydrological variables: discharge, runoff water equivalent, snow depth water equivalent, soil wetness index (root zone). The CEMS-Flood Data User Guide provides detailed instructions on how to download and work with GloFAS forecasts, reanalysis, reforecasts, discharge thresholds.