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1. Introduction
In October 2012, Hurricane Sandy drove a devastating storm surge in excess of 2 m into the northeastern U.S. coastline, tore down trees and power lines that left millions without electricity, and dumped over 900 mm of snow (Blake et al. 2013). As Sandy approached the coast, it acquired structural characteristics consistent with both tropical and extratropical cyclones, with an intact inner-tropical cyclone (TC) warm core embedded within an expansive outer-core wind field (Blake et al. 2013). Contributions from both tropical and baroclinic energy sources caused Sandy to reintensify as it approached the coastline (Galarneau et al. 2013; Shin and Zhang 2017). The TC followed an atypical track northwestward toward the Northeast United States, rather than out to sea, fostered by interaction with an upstream trough (Barnes et al. 2013; Qian et al. 2016) of the type identified by Fujiwhara (1931), the practical predictability of which depended on the modeling system (Bassill 2014; Magnusson et al. 2014; Torn et al. 2015). Sandy tested existing infrastructure for hazard communication (NOAA 2013; Blake et al. 2013) and posed challenges related to risk perception (Meyer et al. 2014) due to its atypical track and forecast structure (Munsell and Zhang 2014) near landfall. Few TCs produce such a broad range of impacts, but Sandy was not ordinary. Rather, Sandy is a dramatic example of the direct impacts, structural evolution, and forecast challenges associated with TCs that become extratropical cyclones, a process known as extratropical transition (ET; Jones et al. 2003).
Tropical cyclones gain energy from warm ocean waters through evaporation and subsequent latent heat release by deep, moist convection. The storm develops a warm core as a result, with the strongest winds near the surface that decrease in strength with height. The wind, precipitation, and temperature fields become more axisymmetric as the TC matures. Conversely, extratropical cyclones are driven by comparatively large temperature and moisture gradients. Within these baroclinic environments, frontal boundaries separate warm, moist air from cool, dry air, resulting in highly asymmetric energy distributions to drive wind and rainfall. In addition, wind speed increases with height due to the cold-core structure of these systems. During ET, the deep warm core associated with the TC becomes shallow and is often replaced by a cold-core, asymmetric structure...