The EFAS medium-range flood forecasts provide an overview of flood probability over the coming 10 days. They are created by comparing the EFAS forecast simulations with EFAS flood threshold levels. These flood threshold levels are calculated for every grid cell, based on a discharge time series simulated by the operational LISFLOOD hydrological model from observed meteorological data.
EFAS medium-range flood forecast products are displayed as colour coded overview maps and time-series information at reporting points that can be viewed under the ‘Flood Summary’ layer of the mapviewer. There are five main products:
- The ‘Reporting Points’ layer (from 8th October 2019; explained below and in 1).
- The ‘Flood Probability’ layers (explained below).
- The 'Flood Probability Persistence' layer (explained below).
- The ‘Threshold level exceedance’ layers (explained below).
- The ‘Rapid Flood Mapping’ and ’Rapid Impact Assessment’ layers.
The Flood Probability Persistence layer shows where on the model river network EFAS forecasts a flood, showing the flood severity level together with the flood probability. The persistence probability map combines the 2-year (yellow), 5-year (red), and 20-year (purple) total (combining all models) exceedance probabilities, averaged from the two most recent forecast runs. It is used in the definition of the reporting points, in a way that no dynamic point is generated below the minimum probability thresholds (50% / 30% / 30%).
Additional local information associated with each flood forecast layer (except for the ‘Flood Probability’ layers) is available from the mapviewer as pop-up plots. They display information such as:
Point geographical information - e.g. country, basin, river and station name.
Forecast summary - e.g. forecast issue date, probability value and tendency and time to peak.
Summary diagrams of catchment meteorological forcings.
Consistency diagrams: show the evolution of the forecast signal over time. They display the forecast threshold exceedance for each return period threshold (1.5-, 2-, 5-, 20-year) for each day of the forecast horizon for the latest eight consecutive forecast runs.
Real-time hydrographs (for certain points only): ECMWF-ENS EFAS forecast post-processed based on observed discharge. They also display the probability of exceeding a mean annual maximum discharge within the next 15 days.
Visit the EFAS wiki page on the reporting points for more information on how to interpret and use reporting points' feature information products.
N.B. EFAS medium-range flood forecasts produced at least 30 days earlier are freely available to view. Real-time EFAS medium-range flood forecasts are only available under certain conditions. Visit the EFAS data access pages to learn more on data restrictions.
1Reporting Point Criteria
The reporting points are created using the following process. For each NWP-driven forecast, a flood probability is computed for each return period flood threshold (2-year, 5-year and 20-year) using the maximum forecast discharge value over the 10-day forecast horizon. This flood probability is only computed on the large rivers with an upstream area > 250 km2. Very dry areas are also excluded by filtering the rivers with a 1.5-year return period < 1 m3/s. A total probability of exceedance is computed from these maximum forecast probabilities (from DWD-HRES, ECMWF-HRES, ECMWF-ENS and COSMO-LEPS). The weights of the forecasts in the total probability are equals but, in order to avoid the predominance of deterministic forecasts in the region where COSMO-LEPS is not available, we impose a ratio of 50% deterministic, 50% probabilistic in the computation of the total probability. This gives the two cases (DWD-HRES, ECMWF-HRES, ECMWF-ENS, COSMO-LEPS):
- Inside COSMO-LEPS domain: (25%, 25%, 25%, 25%)
- Outside COSMO-LEPS domain: (25%, 25%, 50%, 0%)
The persistence probability is then calculated by averaging the total probabilities from the current and the previous forecast. Reporting points are finally created (and their alert levels are computed) based on the following rules:
- 20-year return period: persistence probability > 30% and upstream area > 1000 km2.
- 5-year return period: persistence probability > 30% and upstream area > 1000 km2.
- 2-year return period: persistence probability > 50% and upstream area > 1000 km2.
In regions where no static points (hydrological stations) are available from the EFAS hydrological database, an algorithm is used to generate dynamic points. The algorithm is based on the following properties:
- A dynamic point will be created at the most downstream location and at the peak signal location of a flood signal of at least 5 pixels.
- Once a dynamic point is created, it stays on the map as long as a flood signal exceeding at least the 2-year return period is detected.
- Points too close to each other are removed (points from previous forecasts will remain).
- If two points are close to each other but are on different rivers (based on the upstream area > 1000 km2 network), the two points are kept.