Medicane Zorbas: Origin and impact of an uncertain potentialvorticity streamer

Abstract. Mediterranean tropical-like cyclones (Medicanes) can have high societal impact and their accurate forecast remains a challenge for numerical weather prediction models. They are often triggered by upper-level potential vorticity (PV) anomalies, such as PV streamers and cut-offs. But knowledge is incomplete about their detailed formation processes and factors limiting their predictability. This study exploits a European Centre for Medium-Range Weather Forecast (ECMWF) operational ensemble forecast with an uncertain PV streamer over the Mediterranean, which, three days after initialisation, resulted in an uncertain development of Medicane Zorbas in September 2018. Using an ad-hoc clustering of the ensemble members according to the PV streamer position, it is demonstrated that uncertainty in the initial conditions near an upper-level jet streak over the Gulf of Saint Lawrence is the dominant source of the subsequent uncertainty in the position of the PV streamer over the Mediterranean. The initial condition uncertainty strongly amplifies baroclinically after 18 h in a region of strong quasi-geostrophic forcing for ascent in the left exit of a jet streak over the North Atlantic. The further amplification and downstream propagation of the tropopause-level PV uncertainty leads to a large spread in the position of the PV streamer over the Mediterranean after three days, directly limiting the predictability of the position, thermal structure and evolution of Zorbas. Two low-level airstreams possibly play a key role in linking the uncertainties of the large-scale upper-level flow with meso-scale uncertainties in the cyclone structure. Overall, this study is an illustrative example that uncertainties in large-scale initial conditions can determine the practical predictability limits of a high-impact weather event.