The Extreme Forecast Index for water vapour flux

David Lavers, Ivan Tsonevsky, David Richardson, Florian Pappenberger


In the upgrade of ECMWF’s Integrated Forecasting System to IFS Cycle 46r1, implemented in June 2019, the Extreme Forecast Index (EFI) for water vapour flux became operational. This new EFI parameter can provide an improved understanding of the synoptic-scale processes behind an extreme hydro-meteorological event. In addition, in some cases it can enable earlier awareness of extreme precipitation on the west coasts of mid-latitude continents than the EFI for precipitation.

Following evaluation of the precipitation and water vapour flux EFI across western Europe and western North America, the water vapour flux EFI was found to complement the precipitation EFI by highlighting large-scale water vapour transport in the atmosphere. It was also shown to better identify extreme precipitation in the late medium-range forecast horizon. This is because of its large-scale characteristics and hence higher predictability, which leads to flux predictions being more skilful at these lead times. Conversely, precipitation is linked to smaller-scale processes, such as the cloud microphysics and processes linked with the land surface topography. Precipitation forecasts are therefore less skilful in identifying extremes at these longer lead times. During winter 2018/19, these EFI maps were run in experimental mode and evaluated by users, including the Flood Forecasting Centre based at the UK Met Office and the Atmospheric Rivers Reconnaissance field campaign, following which the EFI for water vapour flux was made operational.

Extreme Forecast Index and Shift of Tails for precipitation and water vapour flux. EFI (shading) and SOT (black contours) in the forecast from 00 UTC on Monday, 22 October 2018 for 72-hour total precipitation and water vapour flux valid from 00 UTC on 28 October to 00 UTC on 31 October 2018. The blue contours show the 500 hPa geopotential at T+180 hours valid at 12 UTC on 29 October 2018. The cyan lines indicate the areas affected by the largest precipitation totals.

An example in Italy

From 27 to 30 October 2018, northern Italy experienced multiple weather hazards including extreme precipitation and flooding. A few stations reported more than 300 mm in 24 hours. This was the result of a large-scale trough over the western Mediterranean, from which a deep cyclone developed and moved from Sardinia to the north. More details on this event can be found in an article by Linus Magnusson and Luigi Cavaleri (Institute of Marine Science, Italy) in ECMWF Newsletter No. 158. In the figure, the EFI maps shown for precipitation and water vapour flux are centred on the event on forecast days 7 to 9. In this case, the precipitation forecast is able to identify the location of the event, as shown by the co-location of high EFI values and the areas with the highest precipitation totals, while the water vapour flux EFI highlights the large-scale nature and atmospheric structure and thus provides a synoptic context for forecast users.

An article by David Lavers et al. published in Weather and Forecasting (doi: 10.1175/WAF-D-17-0073.1) gives more details on the evaluation of the precipitation and water vapour flux EFI across western Europe and western North America.