EFI

An acronym for Extreme Forecast Index (Extreme Forecast Index). It is one of the products produced by the ECMWF and measures the difference between the statistical distribution of the ensemble prediction system (EPS) and that of the model climatology. The EFI can take values between –1 and +1, such that the higher it is, the more unusual the forecast parameter will be. In this sense, we can consider the EFI a "rarity index." Its usefulness lies in the fact that it allows the degree of exceptionality that a given weather episode may have to be quantified.