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The Extreme Forecast Index (EFI) has been developed to provide specialised forecast guidance for anomalous, extreme or severe weather events, (e.g. heavy precipitation, strong winds, heavy snowfall, extreme temperatures, and unusually high ocean waves).  To do this it compares the medium range ensemble forecast probability distribution of an event to the M-climate distribution for the chosen location, time of year and forecast lead time.  The underlying assumption is that, if a forecast is anomalous or extreme with respect to the M-climate, the real weather is also likely to be anomalous or extreme compared to the real climate.   Simple probabilities derived from ENS the ensemble (e.g. the probability that forecast temperature >32°C) do not highlight the differences in the distributions of forecast.  However, EFI will do so because it accounts for the distribution of all the ensemble members.  EFI takes values between +1 and -1.  Experience suggests:

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Ensemble members giving forecast values well beyond the M-climate extreme contribute no more to the EFI than members just matching the M-climate extreme.   The ‘Shift of Tails’ (SOT) is an additional product that has been developed to address this point.  The SOT index complements the EFI by providing information about how extreme an event could potentially be.  Specifically, it compares the tails of the ENS ensemble distribution and M-climate distributionsdistribution.

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