The European Flood Awareness System provides probabilistic flood alert information more than 48 hours in advance to National Authorities. It currently incorporates multiple weather forecasts from three different weather services, real-time weather observations from more than 5000 stations across Europe and real-time hydrological stations from more than 500 stations.

EFAS - A schematic view

Forecast characteristics of the meteorological forecasts used

Hydrological Model

EFAS uses LISFLOOD as a hydrological model. LISFLOOD is a GIS-based, distributed hydrological rainfall-runoff-routing model specifically designed to be used in large scale catchments. Detailed information on its specific characteristics and example applications can be found in the User Manual or in various publications. The model is run in EFAS for all of Europe on 5km grid and either with a six-hourly timestep (Deterministic DWD and ECMWF forecasts, COSMO-LEPS forecasts) or daily timestep (Ensemble ECMWF forecast).

EFAS alert levels

EFAS alert levels are based on a 20 year model run using observed meteorological data as input. A generalized extreme value fitting procedure was then applied to the discharge output maps to obtain return periods for each pixel. The return periods are then associated to EFAS alert levels as shown in the schematic overview and as described below:

EFAS threshold	Colour	Hazard description
S (severe)		Alert level corresponds to a simulated flood event with a return period of >20 yr. Potentially severe flooding expected.
H (high)		Alert level corresponds to a simulated flood event with a return period >5 yr and <20 yr. Significant flooding is expected.
M (medium)		Alert level corresponds to a simulated flood event with a return period >2 yr and <5 yr. Bankfull conditions or slightly higher expected. If flooding occurs no significant damages are expected.
L (low)		Alert level corresponds to a simulated flood event with a return period >1.5 yr and <2 yr. Water levels higher than normal or up to bankfull conditions but no flooding is expected.

EFAS output maps

EFAS displays information as colour coded overview maps and as time-series information at reporting points (circles and triangles). The following output maps are available (background maps are not described here): 

Flood summary layers	Map description
Ongoing floods	Highest level provided exceeded Second highest level provided exceeded Second lowest level provided exceeded Lowest level provided exceeded Single level provided exceeded Stations Available Discharge and Waterlevel Stations Levels Available Stations with Alert Information No data received No data received for stations with Alert Information
Alert level exceed. ongoing	Reporting points where maximum discharge is expected today. Color denotes the highest alert level exceeded by a certain forecast.
Alert level exceed. 1-2 days	Reporting points where maximum discharge is expected within the next 1-2 days. Roman numbers denote when exactly maximum discharge is expected. Color denotes the highest alert level exceeded by a certain forecast.
Alert level exceed. 3-5 days	Reporting points where maximum discharge is expected within the next 3-5 days. Roman numbers denote when exactly maximum discharge is expected. Color denotes the highest alert level exceeded by a certain forecast.
Alert level exceed. >5 days	Reporting points where maximum discharge is expected within the next 5 days or more. Roman numbers denote when exactly maximum discharge is expected. Color denotes the highest alert level exceeded by a certain forecast.
Reporting Points High	Reporting point where the forecasted probability to exceed the EFAS high threshold is > 10 %. Numbers denote the percentage of VAREPS members (left) and COMSO-LEPS members (right), respectively, exceeding the EFAS high threshold.
Reporting Points Medium	Reporting point where the forecasted probability to exceed the EFAS medium threshold is > 50 %.
Flood Probability > 48h	Forecasted flood probability for the forecasting range of 49-240 hours. The flood probability is calculated based on the deterministic DWD and ECMWF as well as the ECMWF EPS forecasts.
Flood Probability < 48h	Forecasted flood probability for the forecasting range of 0-48 hours. The flood probability is calculated based on the deterministic DWD and ECMWF as well as the ECMWF EPS forecasts
Real-time hydrographs	Map showing gauging stations where the forecasts are post-processed in order to 1. minimize the deviance between simulated and measured discharge in real-time and 2. to estimate the predictive  uncertainty The real-time observations are provided by the European Terrestrial Network for River Discharge (ETN-R). On the right the probability of exceeding two thresholds (MHQ and MQ) versus lead-time is shown. The MHQ (shown on the top) is defined as the average yearly maximum value (i.e. from a series of daily discharge data the maximum value per year is selected and the mean of the resulting series of maxima is calculated), whereas the MQ simply means the average of all the discharge values. Therefore the MQ values indicate the discharge under ‘normal’ conditions and the MHQ values correspond to peak discharges which occur every one or two year in the long term average.

EFAS warnings

EFAS sends out warning emails to the corresponding EFAS partners in order to inform them that a possible flood event is upcoming. Two types of emails can be sent by an EFAS forecaster:

• EFAS flood alert: alert issued when a probability of exceeding critical flood thresholds are forecasted more than 2 days ahead in a river basin which is covered with an existing EFAS MoU.
• EFAS flood watch: alert issued when a probability of exceeding critical flood thresholds are forecasted in a river basin for which an EFAS MoU exists but the forecasted event does not satisfy the rules laid out in the MoU, e.g. regarding warning lead time, size of river basin, or location of event. An EFAS flood watch can also be issued if EFAS results are not conclusive but one of the multiple forecasts indicates risk of severe flooding.