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In a sub-seasonal or seasonal forecastsforecast, especially at the the longer ranges, the day-to-day variability of the river flow, with prediction of the actual expected flood severities, can not be expected due to the very high uncertainties. What is possible, is to rather give an indication of the river discharge anomalies and the confidence in those predicted anomalies. As the forecast range increases, the uncertainty will also generally increase and with it the sharpness of the forecast will gradually decrease and forecast will more and more replicate the climatologically expected conditions with some possible positive or negative shift (i.e. anomaly). The generation of the sub-seasonal and seasonal forecast signals and the related products is reflective of this and was designed to deliver a simple to understand categorical information for both the anomalies and the related associated uncertainties present in the forecast, very much relative to the underlying climatological distribution.
The generation of the sub-seasonal and seasonal forecast signal relies on few major steps. The process is illustrated by a flowchart in Figure 1. The forecast anomaly and uncertainty signal is derived by comparing the real time forecast (top left section in Figure 1) to the 99-value percentile climatology. The climatology is generated using reforecasts over a 20-year period, which provides range-dependent climate percentiles that change with the lead time. The climate generation is described in Figure 1, in the bottom right corner. Further details of the real time forecastforecasts, reforecast reforecasts and the generation of the climatologies are available here: Placeholder Description of the real time forecasts, reforecasts and climatologies as components of the CEMS-flood sub-seasonal and seasonal forecasts.
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The revised sub-seasonal products cover calendar week periods (i.e. always Monday-Sunday), while the seasonal products are valid for whole calendar month periods. The forecast signal is derived from the relationship between the calendar weekly or monthly averaged river discharge and the climatological distribution of the same weekly- or monthly-averaged values. While this naturally works for the calendar months, the fixed calendar week weekly lead times in the sub-seasonal allow the users to directly compare forecasts from different forecast runs, as the verification period is fixed onto the calendar weeks. This way, the evolution of the subsequent daily sub-seasonal forecast runs (always at 00 UTC) can be monitored by looking at the exact same verification period.
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Moreover, the overall anomaly of the forecast is defined by the mean of the 51 ensemble member ranks (Rank-mean), which will provide represent a robust anomaly value without any random jumpiness. In addition, the uncertainty about these anomalies is also defined by the 51 ensemble ranks, namely by their standard deviation (Rank-std). Based on this rank standard deviation value, one of three uncertainty categories (low / medium / high) will be assigned to the forecast. Further information on the computation methodology of the forecast anomaly and uncertainty signals is available here: Placeholder CEMS-flood sub-seasonal and seasonal forecast anomaly and uncertainty computation methodology.
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The river network summary highlights the combined forecast anomaly/uncertainty signal on the rivers, including all river pixels above 50 km2 in EFAS and 250 km2 in GloFAS. The forecast signal is indicated by colouring. The colours represent the forecast anomaly and uncertainty categories, with 5 anomaly and 3 uncertainty sub-categories included on the maps, in total with 15 different types of forecast signalcombined forecast signal. Each displayed river pixel will be coloured all the time, as there is always a forecast signal, even a no signal (i.e. perfectly normal condition) is going to be indicated.
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The other major forecast product is the basin summary, which is essentially a geographical summary of the river network summary map, after averaging the signal in the predefined basins and synthesising the more complex and geographically variable information presented on the river pixels in the river network summary layer.
As a new feature of the revised sub-seasonal and seasonal products, both on the EFAS and GloFAS websites, each forecast lead time is plotted individually in the layer. This is accessible through the control panel, which which allows the option to navigate/animate the maps, with the users able to go back and force within the forecast horizon and check each forecast lead time of calendar weeks in the sub-seasonal and calendar months in the seasonal.
The Generally, the product structure and other features are shared between EFAS and GloFAS and similarly between the sub-seasonal and seasonal systems. Further description of these new web products of the river network and basin summary is available here: Placeholder sub-seasonal and seasonal forecast products.