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EFI Charts
The Extreme Forecast Index and Shift of Tails are presented in chart form, either for each parameter separately, or on a composite chart that caters for temperature, precipitation and maximum gusts.
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Multi-parameter interactive charts are available showing the geographical distribution of the EFI of the principal weather parameters: maximum 10m wind gust, 24hr precipitation and 2m temperature, overlaid with the ensemble mean of the MSLP field. This chart highlights areas where there is a significant difference between the ensemble forecast (ensemble) distribution and the model climate (M-climate) distribution.
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The example given in Fig8.1.9.7-10 was associated with a frontal wave system that, although not appearing very dramatic on the synoptic charts, produced significant rainfall as it moved briskly northeastward across parts of northern USA and eastern Canada.
Fig8.1.9.7-10: An example of the use of EFI and SOT during the period leading up to 05-06 September 2018. The evolution of this sequence illustrates the ability of EFI and SOT charts to allow early indication the risk of severe weather (SOT), with an indication of changes in confidence and definition of the area at risk (EFI). EFI tends to be greater than 0.5 if the majority of ensemble member forecast values are markedly greater than those of the corresponding M-climate. EFI>0.5 is coloured (see scales). SOT>0 if at least 10% of ensemble members forecast an extreme event. Isopleths are for values of 0,1,2,5,8. The actual SOT values show how extreme the top 10% of ensemble forecast are.
Chart A (from a forecast 6 days ahead of the event) shows fragmented areas where SOT>0 (i.e. a few ensemble members show an extreme event) so a very severe weather event is possible. However, generally low EFI values (<0.5) shows the majority of ensemble members do not indicate an unusual event (even though a few do).
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Chart D (from a forecast 0 days ahead of the event) shows the areas of SOT>0 and EFI>0.5 are tightly defined, giving confidence to the user in pinpointing the areas at risk. There is an elongated internal area where SOT>1.0, even SOT >2.0, so a very severe event is possible. EFI values are more widely >0.8 (dark orange) so confidence of at least a severe event is growing (for those areas)
Fig8.1.9.7-11: Observed rainfall totals include some above 50mm/24hr and one above 100mm/24hr over eastern Canada during the period 05/12UTC to 06/12UTC September 2018.
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Fig8.1.9.17-14: An example of a forecast of exceptionally calm conditions in the Mediterranean. The chart shows extreme forecast index (EFI) for wave height during the 24 hour period 00UTC 10 Apr 2023 to 00UTC 11 Apr 2023. The colours on the cumulative frequency diagram show the results from the most recent ensemble runs; red is the latest. The forecast cumulative distribution functions (CDF) of the last few runs suggest strongly that wave heights will be only just a few cm. The forecast CDF (red) is well below the M-climate CDF (black) and EFI values are strongly negative.
Differences in EFI over short distances.
At first sight the EFI values shown look unrealistic but can be explained by orography and structure of the atmosphere. The positive (warm) EFI coincides with mountains, whilst the cold EFI in NW Spain is in a relatively high level plateau. It seems the inversion associated with the unusually intense anticyclone (>1040mb) is intersecting the mountains, with highly subsided air delivering higher temperatures where this mountains penetrate upwards. At the same time the lower level cold is fuelled by radiative cooling through the very dry air with light surface winds.
Fig8.1.9.17-15: 24h mean 2m temperature EFI plot for NW Spain 00UTC 18 Dec to 19 Dec 2023, VT 12UTC 17 Dec 2023. Large values for EFI of extreme cold and extreme warm close together .
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