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Tropical cyclones that evolve from a non-tropical origin may pose a specialchallenge for predictions, as they often emerge at the end of a multi-scalecascade of atmospheric processes. Climatological studies have shown that the'tropical transition' (TT) pathway plays a prominent role in cyclogenesis, inparticular over the North Atlantic Ocean. Here we use operational EuropeanCentre for Medium-Range Weather Forecasts ensemble predictions to investigatethe TT of North Atlantic Hurricane Chris (2012), whose formation was precededby the merger of two potential vorticity (PV) maxima, eventually resulting inthe storm-inducing PV streamer. The principal goal is to elucidate the dynamicand thermodynamic processes governing the predictability of cyclogenesis andsubsequent TT. Dynamic time warping is applied to identify ensemble tracks thatare similar to the analysis track. This technique permits small temporal andspatial shifts in the development.  The formation of the pre-Chris cyclone is predicted by those members thatalso predict the merging of the two PV maxima. The position of the stormrelative to the PV streamer determines whether the pre-Chris cyclone followsthe TT pathway. The transitioning storms are located inside a favorable regionof high equivalent potential temperatures that result from a warm seclusionunderneath the cyclonic roll-up of the PV streamer. A systematic investigationof consecutive ensemble forecasts indicates that forecast improvements arelinked to specific events, such as the PV merging. The present case exemplifieshow a novel combination of Eulerian and Lagrangian ensemble forecast analysistool allows to infer physical causes of abrupt changes in predictability.

Tropical transition of Hurricane Chris (2012) over the North Atlantic Ocean: A multi-scale investigation of predictability