A Predictability Pathway from Franklin to Daniel: Linking Extratropical Transition, European Blocking, and Devastating Mediterranean Cyclone Daniel

Abstract. High-impact weather occurring during periods of low forecast skill can lead to severe socio-economic consequences. This study examines the predictability of the causal chain of events leading to the formation of Medicane Daniel and its associated flooding impacts over Greece on 5 September and Northeast Libya on 10–11 September 2023. Daniel’s formation was preceded by Hurricane Franklin’s extratropical transition over the North Atlantic on 1 September 2023, the development of an Omega-type block over central Europe on 3 September 2023, and the detachment of a PV streamer over the eastern Mediterranean which led to Daniel’s formation as a cut-off low on 5 September 2023. Using a novel block error metric, we investigate the predictability of these events using operational ECMWF and Met Office ensemble forecasts and ERA5 reanalysis.

Ensemble sensitivity analysis identifies Franklin’s location as a leading source of downstream uncertainty. Accurate prediction of Franklin’s track was critical for developing an upper-level ridge and European blocking. In ensemble members that best represented the European block, Franklin remained anchored to a downstream ridge, excited a Rossby wave packet over Europe, and initiated a wave breaking event that produced the cut-off leading to the development of Storm Daniel. Two distinct high error recurvature scenarios are identified across the forecasting systems, both of which lead to poor representation of blocking and subsequently to no Mediterranean cyclone development. The strength of Franklin's interaction with the midlatitude flow, and therefore the downstream impact of extratropical transition, is highly sensitive to small track errors at short forecast lead times. Together, these results demonstrate how small upstream errors can contribute to poor predictability of high-impact weather, reinforcing the need for improved representation of the physical processes that take place during tropical–extratropical interactions in operational forecasting to better protect vulnerable regions from impactful weather events.