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Quantile-based weekly guidance maps
Information on the mid-range solution, and the range of possibilities in accessible map-format
Assessment of confidence in the ensemble mean anomaly from climatology
The sub-seasonal ensemble of 100 members runs once per day giving users an early indication of a forecast signal.
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By definition of a median, there is a 50/50 chance of the forecast value of an individual ensemble member being greater or less than the median of the ensemble forecast values. Some of these forecast values may lie within the sub-seasonal climate (SUBS-M-climate), some may lie outside.
Relative spread of ensemble and climatology
It is useful to assess the relative spread of the ensemble forecast against the spread of the sub-seasonal climatology. The inter-decile range (the separation of the 10% and 90% quantiles) is used as this represents the more extreme solutions without considering outliers where sampling maybe an issue.
The relative spread is represented as the ratio of the ensemble forecast inter-decile range divided by the model climate inter-decile range (Fig8.2.9.1).
Fig8.2.9.1: Definition of terms used in deriving a quantile based product. The diagram shows a schematic extract from a meteogram for "Weekly Mean Anomaly of precipitation from Subs-M-climate". Letters denote:
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It is important to note, however, that all members of an ensemble are equally probable and no result should be discarded out of hand.
Fig8.2.9.2(A,B,C): Three examples of the distribution of sub-seasonal ensemble members compared with the sub-seasonal model climate. The shaded colours show range and probabilities of anomalies from the mean in the sub-seasonal model climate. The box and whisker plots show the range and probabilities of anomalies of the sub-seasonal ensemble members from their median. Cumulative distribution functions are shown to help visualisation. If the spread of the ensemble forecast is:
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It is important to note, however, that all members of an ensemble are equally probable and no result should be discarded out of hand.
Examples and usage
Fig8.2.9.3 shows a forecast chart for mean 2m temperature for a period about 3 weeks into the future.In the figure:
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Purple contours (denoting a forecast spread larger than the climatological spread) are relatively uncommon. When present the user should be particularly cautious and be careful not to over-interpret a forecast. Purple contours can appear, for example, as a result of climate-change-related shifts in the cryosphere, or with forecasts of very wet conditions or with a case of severe drought.
Fig8.2.9.3: A forecast chart for mean 2m temperature for a period about 3 weeks into the future.
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