The dynamical interactions between precursor disturbances during the wave-merger cyclogenesis event of 25–26 January 1978 over eastern North America are diagnosed using quasigeostrophic potential vorticity (QGPV) inversion. This case is characterized by two prominent preexisting upper-level disturbances that induce rapid surface cyclogenesis as they come into close proximity. Static QGPV inversion is used to attribute a particular geopotential height field to the QGPV associated with each precursor disturbance. The full flow is partitioned into the following components: the northern upper precursor, the southern upper precursor, and the background flow. Prognostic QGPV inversion is used to quantify the instantaneous geopotential height tendencies attributable to each of these flow components. The static-inversion results for the upper precursors exhibit the structure of baroclinic vortices with maximum amplitude near the tropopause. During the 48-h period spanning the period of study of this event, these vortices rotate cyclonically about a point between them with the rate of rotation increasing as the vortices draw closer together. The background flow appears as a synoptic-scale trough, with the meridional tilt of the trough axis positive (negative) prior to (during) rapid surface cyclogenesis. Prior to surface cyclogenesis, the background flow is also confluent in the vicinity of the vortices, acting to bring them closer together. Rapid surface cyclogenesis occurs as the vortices achieve their closest approach (i.e., “merge”). Three interaction signatures are identified and quantified with prognostic QGPV inversion: vortex–vortex (vortex-induced flows advecting the QGPV of other vortices), vortex-retrogression (vortex-induced flows advecting background QGPV), and back-ground-flow advection of vortex QGPV. Solutions for the observed case confirm that the vortex–vortex interactions become more robust as the vortices come closer together. However, the background advections are dominant and act to bring the vortices closer together. Nearly all of the geopotential height falls at the 1000-hPa cyclone center are due to the advection of the upper precursors by the background flow during the entire cyclogenesis event. A simple model is proposed that includes the three primary elements of this case: two vortices and a background flow. For a barotropic atmosphere on an f plane, the vortices are represented by rigid vortex patches and the background flow by a hyperbolic deformation field that is fixed in time. Solutions representative of observed parameters exhibit many of the properties of the observed case, including “merger.” Solutions corresponding to merger are found to be extremely sensitive to small changes in the deformation field for a given set of initial conditions describing vortex position, size, and strength, suggesting limitations to the predictability of the merger phenomenon.