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In the EFI scheme, the probabilities are
compared to the model climate (M-climate) distribution for the
chosen location, time of year and lead time. The underlying
assumption is that, if a forecast is anomalous or extreme, relative
to the M-climate, the real weather is also likely to be anomalous
or extreme compared to the real climate.
The M-climate is based on five
consecutive weekly 32-day re-runs of the current EPS model, with
four perturbed and one unperturbed initial conditions and started
from ERA-Interim re-analyses from each of the last 18 years. The
resolution decreases with forecast range exactly as in the
operational EPS. This procedure allows seasonal variations and
model changes to be taken into account, as well as model
drift. An additional advantage of the EFI is that it compensates
for areas where the ECMWF model climate is less accurate, such as
in the tropics.
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The model climate for the
EFI calculations on Sunday 31 October 2010 at 12 UTC is, for
example, prepared from five forecast runs from five Thursdays,
centred on the 28 October, i.e.14, 21 and 28 October and 4 and 11
November, for all the 18 years, totalling 450 forecasts
(see
Figure 50).
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