The Radar Coverage product shows the area covered by the OPERA pan-European radar composite for a specific forecast. The shadings show the relative weight given to the radar vs the ECMWF data, where darker greys mean more weight was given to the ECMWF forecast data (Figure 1). The radar data are used to derive rainfall accumulation nowcasts which are input to the radar-based flash flood products, and therefore users are encouraged to use the radar coverage product to help interpret the radar-based flash flood impact products. For example, the urban flash flood products are not be available in areas outside of the radar coverage, and river flash flood impacts are forecasted in areas outside of the radar coverage using forecasts from the ECMWF NWP, expected to be less accurate in predicting extreme rainfall associated with convective conditions and only updated 4 times per day rather than every hour. We recommend to exercise caution when interpreting the river flash flood impact products in areas outside of the valid radar coverage.
Figure 1. Radar coverage product shown on the EFAS web interface for the 19th February 2025 at 00 UTC. The darker grey shades show where no radar data were available.
Methodology
The radar data which are provided at a given time include a mask, which shows locations within the European domain (Figure 1) with no valid data from the radar observation network. Radar data can be missing for reasons including: the area not being covered by the existing radar network, the radar at a particular location being offline at that particular time, or errors with the data provided by a particular radar location.
A spatial weighting is applied to blend between the radar and ECMWF NWP data, this is to prevent the appearance of sharp boundaries. This weighting gives increasing weight to the ECMWF NWP data with increasing distance from the nearest radar station location. Typically radar data given zero weighting beyond a distance of 100 km from the nearest radar station. The shading of the radar coverage product on the EFAS web interface shows the weighting given between the radar and ECMWF data, darker grey shades means more weight is given to the ECMWF forecast data.
Layer Usage on the EFAS Web Interface
When the layer is loaded into the EFAS web interface, if no other layers are loaded, it will by default load for the latest 00 or 12 UTC forecast for which the main hydrological EFAS forecasts are available. If the user wishes to view the layer for a hourly forecast, i.e. one other than 00 or 12 UTC, the user must do the following:
- Remove the Radar coverage layer from the legend window
- Load the Radar-based river flash flood impact layer
- Select the forecast date of interest
- Load the Radar-based coverage layer