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Table 1: Definition and description of the 7 anomaly categories. The possible value ranges in the 'Ranks Rank column' are inclusive at the start and exclusive at the end, so for example for Cat-1 the category of Extreme low the possible ranks are 1, 2, 3, ... and 10. For some web products, the middle three categories (Cat3, Cat-4 and Cat-5Bit low, Normal and Bit high) are combined into one extended 'Near normal' category. The categories are colour coded as they appear on the web products.
Extremity rank computation for ensemble members
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Based on these specific examples and the rank-std ranging from 0 to 49.5, with the climatology-matching distribution's ranks-std of 29, three uncertainty categories were defined, using the easy to remember generic category split values of 10 and 20. (Table 2). These work well enough for all lead times and geographical areas and will give an indication of generally how uncertain the expected forecast anomaly is for that pixel or area and that lead time.
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Table 2: Uncertainty categories defined by the standard deviation of the ensemble member ranks. The categories are also colour coded as they appear on the web products for the 'Near normal' extended anomaly category.
Simplified forecast examples to help interpreting the expected anomaly and uncertainty category information
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