Use of ensemble forecasts to investigate synoptic influences on the structural evolution and predictability of Hurricane Alex (2016) in the midlatitudes

Hurricane Alex was an extremely rare hurricane event, the first North Atlantic hurricane to form in January since 1938. Alex developed from an extratropical low pressure system that formed over the western North Atlantic basin, and then underwent tropical transition after moving to the eastern basin. It subsequently underwent anomalous extratropical transition (ET) just north of the Azores Islands. We examine the factors affecting Alex's structural evolution and the predictability of that evolution. Potential scenarios of structural development are identified froma 51-member forecast ensemble fromthe European Centre forMedium-Range Weather Forecasts Ensemble Prediction System (ECMWF-EPS), initialized at 0000 UTC 10 January 2016. The EPS forecasts are clustered using a regression mixture model based on the storm's path through the cyclone phase space. Composite maps constructed from these clusters are used to investigate the role of synoptic-scale features on the evolving structure of Hurricane Alex as it interacted with the midlatitude flow. Results suggest that the crucial factor affecting this interplay was the behavior of a large extratropical cyclone and its associated cold front and likely warmconveyor belt upstreamofAlex; the intensity of these structures determined whether Alex underwent a typical cold-core ET (as observed) or a warm-seclusion ET. The clustering and compositing methodology proposed not only provides a nuanced analysis of the ensemble forecast variability, helping forecasters to analyze the predictability of future complex tropical-midlatitude interactions, but also presents a method to investigate probable causes of different processes occurring in cyclones.