The ECMWF Extreme Forecast Index for Water Vapour Transport

At the European Centre for Medium-Range Weather Forecasts (ECMWF), one index that summarises the forecast information from the Integrated Forecasting System ensemble (ENS) is the Extreme Forecast Index (EFI). The EFI provides a way of highlighting those regions that are forecast to have possibly anomalous weather conditions compared to the local climate. By identifying these areas, early awareness may be provided of, for example, extreme precipitation or temperature.

This research introduces the EFI for water vapour transport (integrated vapour transport IVT) and undertakes a verification across different regions throughout the medium-range forecast horizon (out to 15 days) on the ability of the EFI for IVT and precipitation to capture extreme observed precipitation. The IVT is a useful diagnostic for atmospheric rivers, a region of intense moisture flux within extratropical cyclones, and has been shown to cause heavy precipitation and flooding on the west coasts of mid-latitude continents.

Using the ECMWF ENS for winters 2015/16 and 2016/17 and daily surface precipitation observations, the relative operating characteristic is used to show that the IVT EFI is more skilful than the precipitation EFI in the late medium-range over Europe and western North America. The large-scale characteristics of IVT, its higher predictability, and its connection with extreme precipitation result in its potential usefulness, which could provide earlier awareness of extreme precipitation in these regions. Conversely, at shorter lead times the precipitation EFI is more useful, although the IVT EFI can provide synoptic-scale understanding. For the whole Globe, extratropical Northern Hemisphere, Tropics, and North America, the precipitation EFI is more useful throughout the medium-range suggesting that precipitation processes not captured in the IVT are also important.