This page describes the way the anomaly signal and uncertainty of the ensemble forecasts in the sub-seasonal and seasonal products are determined using the climatology as reference. This includes also how the expected forecast anomaly category (amongst 5 or 7 pre-defined categories) and the uncertainty category (divided in 3 categories low/medium/high) of the ensemble forecasts are determined. This is a generic procedure, which is the same for both EFAS and GloFAS, as it is executed the same way for each river pixel, regardless of the resolution, and also the same for the sub-seasonal and seasonal products, as it works in the exact same way regardless of whether it is weekly mean values, as in the sub-seasonal, or monthly mean values, as in the seasonal.
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From the climate sample, 99 climate percentiles are determined, which represent the range of equally likely (1% chance) segments of the river discharge value range magnitude that occurred in the 20-year climatological sample (both sub-seasonal and seasonal is currently based on 20 years). Figure 1 shows an example generic climate distribution, either based on weekly means or monthly means, with the percentiles represented along the y-axis. Only the deciles (every 10%), the quartiles (25%, 50% and 75%), of which the middle (50%) is also called median, and few of the extreme percentiles are indicated near the minimum and maximum of the climatological range shown by black crosses. Each of these percentiles have an equivalent river discharge value along the x-axis. From one percentile to the next, the river discharge value range is divided into 100 equally likely bins (separated by the percentiles), some of which is indicated in Figure 1, such as bin1 of values below the 1st percentile, bin2 of values between the 1st and 2nd percentiles or bin 100 of river discharge values above the 99th percentiles, etc.
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