An analysis is our best estimate of reality, used as a proxy for observations which do not exist everywhere. It is the archived lead time 0 of a forecast. For example, the archived water balance of EFAS is an analysis.

Archived forecast

Archived forecasts are past forecast runs; these differ in that they are not re-run when the model changes, so the model version is not consistent throughout the time period.


Set of reference simulations, including historical simulations forced with observations or reanalysis.

Control forecast

Ensemble forecast sub-set only including simulation obtained from the control member of the corresponding Numerical Weather Prediction (NWP). The control member is the unperturbed forecast run at the resolution of the ensemble forecast. Recommended to be used in combination with the perturbed forecasts.


COnsortium for Small-scale MOdelling Limited-area Ensemble Prediction System. Probabilistic meteorological forecast (ensemble forecast) of 20 members, i.e. model simulations, with the COSMO model covering the European domain. The spatial resolution is ~7 km. Within EFAS, COSMO-LEPS is used to produce a probabilistic flood forecast for Formal and Informal Flood Notifications as well as for calculation of the ERIC Flash Flood indicator that is based on this probabilistic flood forecast.


Deterministic forecast that uses the COSMO-EU model for the first 3 days, then the ICON model for days 4-7. The spatial resolution for the first 3 days of the forecast is ~6.5 km, for days 4-7 13 km.


High-resolution forecast (HRES) using the ECMWF meteorological model. It has a maxmimum lead time of 10 days and a spatial resolution of ~9 km.


Probabilistic meteorological forecast (ensemble forecast) consisting of 51 members run with the ECMWF model covering the European domain. The spatial resolution was ~18 km until the 26.06.2023. Since the 27.06.2023 the spatial resolution is ~ 9 km.

One forecast, called the control forecast, is run from the operational ocean and atmosphere ECMWF analyses. 50 additional integrations, the perturbed members, are made from slightly different initial atmospheric and oceanic conditions, which are designed to represent the uncertainties inherent in the operational analyses. Within EFAS, ECMWF-ENS is used to produce a probabilistic flood forecast for Formal and Informal Flood Notifications.


EFAS Information System. It is the Map Viewer interface that can be accessed from EFAS homepage and that contains all layers necessary for the analysis of the forecasts.

EFAS Map Viewer

Another term for the EFAS-IS, an interactive map for analyzing EFAS forecasts.

Ensemble forecast (ENS)

A number of simulations (members) using the same NWP model, but varying the initial conditions and/or the tendencies. The ensemble members are perturbed (pushed away from the observed state) with regards to the best estimate of the initial conditions, and the ensemble spread should ideally reflect the model error. Ensemble forecasts are often used to express the uncertainty of a forecast by associating a probability with different forecast values.

Ensemble Streamflow Prediction (ESP)

A forecasting method consisting of starting a hydrological model with the latest initial hydrological conditions (e.g., soil moisture, river level, snow cover, etc) and forcing it with historical meteorological observations or reanalysis (e.g., precipitation, temperature, evaporation, etc). Each ensemble member will thus correspond to a year of historical meteorological observations. For example, to generate hydrological forecasts on the 1st of February 2016 for the next 7 months of lead time, the hydrological model would be started with the hydrological conditions observed on this day. Meteorological forcing would then be taken from each available year of meteorological observations for the 1st of February - 31st August (+7 months).


Emergency Response Coordination Centre (ERCC) of the European Commission’s Directorate-General for European Civil Protection and Humanitarian Aid Operations (DG ECHO). The daily analysis of the EFAS forecasts is sent to the ERCC.


European Runoff Index based on Climatology used as flash flood indicator. Within EFAS, ERIC is generated by comparing the forecasted surface runoff accumulated over the upstream catchment with a reference threshold. It is based on the 20-member COSMO-LEPS ensemble precipitation and soil moisture forecasts from the LISFLOOD hydrological model and provides indicators for the next 5 days for catchments smaller than 2,000km2.

Extended-range forecast

In the ECMWF context it is the ensemble forecast with a lead time of 46 days. It is issued bi-weekly (Mondays and Thursdays) at 00UTC.

Flash Flood Notification

An EFAS Flash Flood Notification is sent out to the affected EFAS partner if the probability of exceeding the 5-year return period of the ERIC indicator is equal or greater than 30%.  Additionally, the lead time to the start of the event, i.e. the time lag between the start of the model run and the point where 30% of the probabilistic forecasts exceed the 5-year return period threshold, may not be longer than 48 hours. Flash Flood Notifications are sent for catchments smaller than 2000 km2.

Forced simulation

Hydrological time series simulation forced with observations, reanalysis or other proxy for observations. Typically used to generate initial conditions for a hydrological forecast and reference long-term run. The latter can be used to calculate return periods and for validation of forecasts.

Formal Flood Notification

An EFAS Formal Flood Notification is sent out to the affected EFAS partner if one of the two probabilistic and one of the two deterministic forecasts is persistently over the 5 year return period level for three consecutive model runs and if the start of the event is more than 48 hours from the run time of the model. Formal Flood notifications can only be issued if the catchment area is larger than 2000 km2.

High-resolution forecast (HRES)

Single simulation with one meteorological forecast input. The high-resolution NWP input is usually better than ensemble forecasts in resolving local features, such as orographic precipitation, but small deviations in the model simulation, for example, geographical displacement, can cause large deviations in the hydrological output in comparison with observations.


Hindcast is another commonly used term for reforecast; while these mean the same thing, hindcast is used less and less to be replaced with reforecast.

Historical simulation forced with observations

Long (> 20 year) hydrological time series simulation forced with proxy-observed meteorological fields (e.g. reanalysis such as the ERA5 product). Typically used to generate reference statistics, for example, return period discharge magnitude against which to compared forecast simulations to generate warnings, and to validate hindcasts.

Informal Flood Notification

An EFAS Informal Flood Notification is issued when any of the criteria valid for EFAS Formal Flood Notifications is not met, but the event still is worth mentioning.

Lead time

The length of time between the issuance of a forecast and the future time step one is trying to predict (e.g. the time step the event peaks).


The hydrological model used within EFAS. LISFLOOD is a GIS-based, distributed hydrological rainfall-runoff-routing model specifically designed to be used in large scale catchments.

Medium-range forecast

Forecasts produced for a lead time of several days (typically 3-15 days). The HRES and the ENS constitute medium-range forecasts.

Numerical weather prediction model (NWP)

Mathematical models of the atmosphere, oceans, and land processes that predict the weather based on current weather conditions.

Perturbed forecast

Ensemble forecast sub-set excluding its control member. Generally used in combination with the control forecast. The order of the numbering of the perturbed forecasts is random.


A reanalysis is the same as analysis but produced using a frozen model version giving the benefit that the model has not changed over the time period of the reanalysis (note that the observations assimilated will still change over time depending on what was available).


Forecast simulations performed for a set of past dates, based on a configuration as close as possible to the operational setting. They are used to evaluate the forecast skills against a benchmark or reference forecast.

Seasonal-range forecast

Forecasts produced for a lead time of several months (typically 30 days to 7 months)

Time step

Time increments for each forecast. A time step of 6 hours means that the forecast as a value every 6 hours until the maximum forecast horizon.

Upstream areaThe total area that contributes with water to a specific point on the river network. Also known as a catchment area.