  Tropical cyclones that evolve from a non-tropical origin may pose a special
challenge for predictions, as they often emerge at the end of a multi-scale
cascade of atmospheric processes. Climatological studies have shown that the
'tropical transition' (TT) pathway plays a prominent role in cyclogenesis, in
particular over the North Atlantic Ocean. Here we use operational European
Centre for Medium-Range Weather Forecasts ensemble predictions to investigate
the TT of North Atlantic Hurricane Chris (2012), whose formation was preceded
by the merger of two potential vorticity (PV) maxima, eventually resulting in
the storm-inducing PV streamer. The principal goal is to elucidate the dynamic
and thermodynamic processes governing the predictability of cyclogenesis and
subsequent TT. Dynamic time warping is applied to identify ensemble tracks that
are similar to the analysis track. This technique permits small temporal and
spatial shifts in the development.
  The formation of the pre-Chris cyclone is predicted by those members that
also predict the merging of the two PV maxima. The position of the storm
relative to the PV streamer determines whether the pre-Chris cyclone follows
the TT pathway. The transitioning storms are located inside a favorable region
of high equivalent potential temperatures that result from a warm seclusion
underneath the cyclonic roll-up of the PV streamer. A systematic investigation
of consecutive ensemble forecasts indicates that forecast improvements are
linked to specific events, such as the PV merging. The present case exemplifies
how a novel combination of Eulerian and Lagrangian ensemble forecast analysis
tool allows to infer physical causes of abrupt changes in predictability.
