This layer forecasts 1-arcminute (~1.4 km) river grid cells where there is a risk of impacts due to river flash flooding, which refers to the rapid rise of small rivers due to intense rainfall. The forecasts are generated every hour using a blending of ensemble precipitation forecast inputs derived from the latest radar observations and NWP forecasts from the ECMWF ensemble. The forecasts have a 1-hour time step for the first ~8-hours of the forecast, and a 6-hour time step thereafter up to a maximum lead time of 120-hours (5-days). Flash flood impacts are categorised on an impact matrix as being either low (yellow), medium (orange), high (red) or severe (purple), river grid cells affected by flash flooding will be highlighted in one of these four colours (Figure 1). When this layer is loaded on the EFAS web interface, it is possible to use an animation slider (which appears in the bottom left corner) to visualise each time step of the forecast. Please note, currently it is only possible to view this layer for the current and previous 5 days, this is due to the technology which is used to load the layer on to the web interface.
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Figure 1. The Radar-based river flash flood impact layer shown on the EFAS web interface for the Dublin area on 4th September 2022 at 04:00 UTC (left), and the impact matrix (right) which is used to assign the colour of each river grid cell affected by flash flooding.
Methodology
There are main steps in the generation of these four river flash flood impact catchments summary layers: 1) river flash flood hazard forecasting using blended radar and NWP precipitation, 2) river flash flood impact prediction (Figure 2).
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