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- 0-6 hours (based on blending radar precipitation nowcasts and the ECMWF ensemble weather forecast)
- 7-24 hours (based only on the ECMWF ensemble weather forecast)
- 25-48 hours (based only on the ECMWF ensemble weather forecast)
- 49 -120 hours (based only on the ECMWF ensemble weather forecast)
This The forecasted impact level in each river catchment is shown as one of four levels: low (yellow), moderate (orange), high (red) and severe (purple). The impact level is estimated by using a risk matrix to intersect a flash flood hazard forecast with static exposure data of population and critical infrastructure. Both the flash flood hazard and the exposure data are split into three categories to create the risk matrix (see an example in the left hand side of Fig. 1).
Methodology
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There are three 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, 3) summarising river flash flood impacts at the catchment scale.
Step 1: River Flash Flood Hazard Forecasts
The low, medium, and high values for flash flood hazard on the y-axis of the impact matrix indicate where there is a 5%-50%, 50%-80%, and >80% probability of exceeding the 2-year return period threshold. This flash flood hazard is estimated by comparing blended forecasts of precipitation (from the OPERA radar mosaic and ECMWF NWP forecasts) accumulated on the river network, with the reference values derived from climatology. This climatology is from:
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