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The most consistent way to convey forecast uncertainty information is by the probability of the occurrence of an event.  The event can be general or user-specific regarding probability of exceeding an event threshold.  The event threshold may correspond to the point at which the user has to take some action to mitigate potential damage from a significant weather event.   Probabilities can be:

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Forecast intervals (e.g. “temperatures between 2°C and 5°C”, or “precipitation between 5 and 8mm/24hr”) can be used as a hybrid between categorical and probabilistic forecasts.  ecCharts provide a simple way of displaying probabilities above or below thresholds and by intercomparison can give a indication of probability of a parameter lying between the thresholds.   For example for maximum temperatures at Vilnius, (see Fig8.1.1.10) there is a 20% probability of being ≥20°C, and from Fig8.1.1.11 there is a 25% probability of being ≤15°C.  Therefore there is a 55% probability that the maximum temperature will lie between these two values.  Combinations of parameters are possible (e.g. the probability of combined events of wind gust and total snowfall is available on ecCharts as an aid to forecasting drifting of snow).

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Fig8.1.1.10: Chart taken from ecCharts showing ensemble probability of maximum 2m temperature ≥20°C (ecCharts colour bands for this scheme denote 55%-2020%-4040%-6060%-8080%-9595%-100%). There  There is a 20% probability of maximum temperatures ≥20°C at Vilnius (shown in the box). The  The location of Vilnius is shown by the pin.

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Fig8.1.1.11: Chart taken from ecCharts showing ensemble probability of maximum 2m temperature ≤15°C (ecCharts colour bands for this scheme denote 55%-2020%-4040%-6060%-8080%-9595%-100%).  There is a 25% probability of maximum temperatures ≤15°C at Vilnius (shown in the box).  The location of Vilnius is shown by the pin.

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Several charts on ecCharts are available to show the probability of combined events with thresholds under user control.   The  The probability is computed as the ratio of the number of the ensemble members in which both event conditions are met to the total number of ensemble members.  The current available charts are probabilities of:

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Fig8.1.1.12: Chart taken from ecCharts showing ensemble probability of wind gust ≥10m/s and ≥2mm/12hr (ecCharts colour bands for this scheme denote Light blue 5-35%, Blue 35-65%, Dark blue 65-95%, Purple >95%).  There is a 31% probability of exceeding the thresholds at Munich (shown in the box).  The location of Munich is shown by the pin.

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