Dynamics of forecast-error growth along cut-off Sanchez and its consequence for the prediction of a high-impact weather event over southern France

The representation of a high-impact weather (HIW) event over southern France is evaluated in Météo-France forecasts, and the sensitivity of the HIW forecast to the upstream upper-level flow and the Mediterranean and North Atlantic humidity structure prior to the event is quantified. The event occurred in October 2016 during the international field experiment NAWDEX. The approach of an upper-level potential vorticity (PV) cut-off, referred to as cut-off (Formula presented.), triggered extreme precipitation over southern France. Many 2- to 7-day ensemble forecasts predicted the maximum of the extreme precipitation and the location of the upper-level PV cut-off too far to the east. This eastward shift primarily resulted from an underestimation of the cut-off intensity two days before the HIW and the subsequent downstream propagation and amplification of these errors in the vicinity of (Formula presented.). Improving the representation of the cut-off two days before the event significantly improved the forecast quality. Another error source were inaccuracies in the moisture structure in the eastern North Atlantic. Specifically, an underestimation of the moisture in the warm conveyor belt inflow led to errors in the low- and upper-level circulation that eventually contributed to the eastward shift of the HIW two days later. Corrections in the eastern North Atlantic humidity structure further improved the forecast quality. On the other hand, corrections in the Mediterranean humidity structure had only a small impact on the accuracy of the forecast. The findings illustrate the importance of downstream error propagation and moist diabatic processes for the prediction of extreme weather over Europe, and demonstrate how targeted changes in the PV and humidity field a few days in advance can improve the quality of the forecasts.