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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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.

Impact of local time differences from UTC when using EFI and SOT charts

EFI and SOT charts and diagrams refer to 24hr periods (T+00 to T+24, T+24 to T+48, T+48 to T+60 etc, etc.) and the charts run from 00UTC to 24UTC  each day.   Thus, depending on the longitude, the period of the 24hr EFI and SOT will not match local time and charts of EFI probability may span over two days in local time.  So as examples:

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Severe storms may appear on two consecutive charts (e.g. afternoon local time severe storms in USA continuing through the evening local time before decaying will appear on two consecutive days but possibly with a lower probability in each period.  It is important to cater for the time differences when considering the probability of extreme weather.

EFI Web open access charts

To view Medium Range CDFs:

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Composite Multi-parameter Interactive (clickable) EFI charts

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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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

ecCharts and Dashboard Widgets

For a location chosen by the probe tool on ecCharts or by selection of a widget on the dashboard:

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Cumulative Distribution Function Diagrams

Attached to each grid point of the global EFI charts there is a CDF diagram for each of the EFI parameters, with information on M-climate at the grid point (always shown for the lead time 24-48h) and the available forecast distributions (all valid for the given day, but coming from different, consecutive ensemble runs, so the lead times vary).  On the right are shown the corresponding EFI values.  These diagrams can be displayed interactively by clicking on the desired location on the EFI web charts,  use of the probe tool on ecCharts, or selection of the widgets on the dashboard.

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Extreme Forecast Index (EFI) and Shift of Tails (SOT) versus Lead Time

The development and evolution of EFI and SOT over a sequence of charts can give an early insight into the likelihood of occurrence of out-of-the-ordinary weather and the confidence that may be assigned to developments.  EFI values roughly indicate the extent to which ensemble members forecast values exceed the corresponding M-climate.  At longer lead-times the number of such members may be quite low. However areas of SOT >1 on longer lead charts give an indication that at least some ensemble members are showing extreme values.  This shows there is a risk, albeit small, of out-of-the-ordinary weather within the marked area.  As the forecast lead time shortens, EFI values will indicate any increase or decrease in the number of members exceeding the corresponding M-climate value.  At the same time the isopleths of SOT will show the variation in the potential for extreme values and give an increasing definition of the risk area (if there is one!).

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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.

Very high uncertainty illustrated by EFI and SOT

The ensemble is designed to give an assessment of uncertainty.  However, occasionally the uncertainty becomes very large, with additionally a potential for very high or low forecast values to occur.  An example is shown in Fig8.1.9.7-12 where ensemble members showed widely differing yet potentially extreme hot or cold temperatures.  These cases are relatively rare, usually associated with a sharp upper trough and associated front separating widely dissimilar airmasses.  Users should accept the uncertainty and tailor their forecasts to reflect the impact on users.  Certainly they should not take middle course.  Some customers may require an indication of very high or very low temperatures, even if at a low probability. 

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

An example of high probability of an extreme event.  Rainfall in the desert.

Fig8.1.9.17-13:  Extreme Forecast Index for total precipitation during the 10 day period 00UTC 10 Apr 2023 to 00UTC 20 Apr 2023.   The forecast cumulative distribution function (CDF) suggests that during the period there is less than 10% probability of below 35mm precipitation, 90% probability of below 80mm precipitation, and 10% probability of greater than 80mm.  The forecast CDF (red) is well above the M-climate CDF (black) and shift of tails values are quite high.  For a desert region, quite moderate precipitation values can be devastating.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

An example of high probability of an unusual event.  Very calm sea in the Mediterranean.

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.  

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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.  

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Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• Read more about EFI in seasonal forecasts (EFI in seasonal forecasts is not available operationally).
• Watch a comprehensive lecture on forecasting extreme events using EFI and SOT.

(FUG Associated with Cy49r1)