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→ This is because the climatological distribution of 7-day rainfall is skewed in most locations, with the median having a lower value than the mean. So across the world as a whole, for weekly rainfall, below average is ordinarily more likely than above average (where average is referring to the mean). As colour shades represent the (forecast) median minus the (climatological) mean, the default behaviour, if the medians of the forecast and climate distributions are about the same (which is much more likely at longer leads) will be to show a "dry" shade, reflecting the fact that dry is genuinely more likely. This allows the user to correctly put into words a probabilistic outcome, in some absolute sense, even if the forecast signal itself is very muted. Unlike some other forecast products these quantile-based maps are strongly user-oriented, combining in this case two types of information content - local climatological characteristics and forecast signal. A researcher might on the other hand might be particularly interested in just identifying where and when there is a probabilistic forecast signal that differs from climatology; although these products do provide a lot of insights into that aspect, for a completely clean view they would need to use something different.
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→ As an aid to learning, and to better comprehend the overall signal shown by the two layers, we recommend that (new) users click on a feature of interest on a product to bring up a meteogram, and then use the drop-down product menu (top left) for that meteogram to select "Sub-seasonal Anomaly meteogram". The medians and the 10th and 90th percentiles can then be compared for the selected location, between the forecast and the model climate.
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