Aided with a numerical simulation, the authors diagnose the synoptic-scale evolution of an intense extratropical cyclone over the western Atlantic Ocean from the perspective of balanced dynamics using potential vorticity inversion techniques. The authors show that even during the period of largest growth rate, early in the life cycle, the balance equations capture much of the nondivergent wind, irrotational wind, and height tendency fields. The structure of the cyclone evolves from a maximum amplitude at the tropopause to a maximum amplitude at the surface as the system moves offshore. The vertical phase structure remains slightly upshear tilted throughout (less than one-quarter wavelength). Cyclogenesis is interpreted in terms of a baroclinic wave propagating through a varying basic state, in which surface development is rapid because of the sudden response of the wave structure to a changing environment. The mature cyclone structure strongly resembles cases of continental cyclones except that the relative contribution of condensation-produced PV anomalies dominates the contribution from the lower-boundary thermal anomalies.