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 12~8-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 (left), and the impact matrix (right) which is used to assign the colour of each river grid cell affected by flash flooding.
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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).
Figure 2. The impact matrix upon which flash flood hazard probability (y-axis) is combined with exposure (x-axis) to decide the impact category as shown by the colour of each cell. The definition of each impact category is shown on the right hand side.
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The layer can be viewed after selecting it from the Flash Flood layers tab within the EFAS webviewer. An animation slider box will appear in the bottom left of the screen (Fig. 2Figure 3) and by default the data from the most recent forecast will be loaded. A drop down menu just above the animation slider box can be used to select a different forecast date and time within the past 5 days.
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The animation slider box is shaded in white for the first 6 8 hours of the forecast horizon, this represents the horizon during which radar nowcasts are blended with the ECMWF ensemble NWP. For the remaining timesteps the animation slider box is shaded red, this reflects that the forecast is derived from ECMWF ensemble NWP only.
For the first few hours of the forecast horizon the timestep is of the forecast is 1 hour, after this the timestep becomes 6 hourly. Typically the first 6 hours of the forecast will have a 1 hour timestep, but this could be longer depending on the time of day when the forecast was produced. A box is displayed above the animation slider which shows the size of the current timestep which is being shown in the animation slider.
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3. Flash flood impact forecast for the Dublin area on 4th September 2022 at 04:00 UTC
Suggested Workflow for Analysing Animated Flash Flood Impact Layer
When viewing the animated TAMIR Radar-based river flash flood impact forecasts layer it can be difficult to identify specific locations and timesteps which may be affected by flash flooding. This is partly because when zoomed out it is very difficult to see the shaded 1 km -arcminute grid cells. To solve this, the following workflow is recommended:
- Load in the TAMIR Catchment Summary Radar-based river flash flood impact catchment summary layer for the forecast leadtime horizon window of interest e.g. load in the TAMIR Radar- Impact Max based River Flash Flood Impact Catchment summary 0-6 Catchment Summary 6h if you are interested in the first 6 hours of the forecast leadtime horizon
- Use this layer to identify the specific sub-catchments which are highlighted as being at risk of flash flooding during this forecast horizon
- Zoom in closer to the highlighted subcatchments of interest
- Load in the TAMIR - impact forecasts Radar-based river flash flood impact layer, use the animation slider to identify the specific 1 km grid cells and timesteps where flash flood impacts are possible
The visibility of the TAMIR - impact forecasts Radar-based river flash flood impact layer can be improved by changing the map background to show country borders only, this can be toggled from the top right of the webviewer, it is the icon to the right of the WMS icon