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Although higher EFI values indicate that an
extreme event is more likely than usual, the values do not
represent probabilities, as such. Any forecasts or warnings must be
based on a careful study of probabilistic and deterministic
information.
Since potentially extreme situations (wind
storms, for example) are characterized by high dynamical
instability in the atmosphere and high EPS spread, EFI users should
be aware that it is not uncommon for an extreme event to be
preceded by wide-ranging shallow slope CDFs, yielding EFI values
that are not particularly high. CDFs should be directly referenced.
If, for example, the EFI indicates to forecasters that anomalous
wind speeds or rainfall rates are more likely than normal, they
have to find out from the CDF diagram what this means for a
specific threshold, e.g. 5 mm/12 hours. If the climatological risk
is 5% and the predicted probability is 20%, the risk is four times
larger than normal. Any action will, however, depend on whether
this 20 % is high enough for a specific end-user to undertake
protective action.
Finally, another key issue of the EFI is
that members well beyond M-climate extremes contribute no more to
the EFI than members matching the M-climate extreme. Although the
EFI index is a useful tool that allows the easy identification of
extremes with respect to location and season, its simplicity is
achieved by a rather complex mathematical treatment and should only
serve as an “alarm bell”, a warning of potentially
extreme events.
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