Major river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 100 mm of rain to a large swath of northern England and north Wales, with local accumulations approaching 200 mm. The cyclone developed on 20–22 September following the interaction between an equatorward-moving potential vorticity (PV) streamer and Tropical Storm Nadine, near the Azores. A plume of tropical moisture was drawn poleward ahead of the PV streamer over a low-level baroclinic zone, allowing deep convection to develop. Convectively driven latent heat release reduced upper-tropospheric PV near the streamer, causing it to fracture and cut off from the reservoir of high PV over the United Kingdom. Simulations using the Weather Research and Forecasting Model with 4-km horizontal grid spacing in which microphysical heating and cooling tendencies are set to zero, alongside calculations of instantaneous diabatic heating rates and PV tendencies along trajectories, reveal that deposition heating contributed strongly to the fracturing of the PV streamer into a discrete anomaly by directly reducing upper-tropospheric PV to the streamer’s east. Condensation heating contributed to lower-tropospheric PV generation along the cold front as the cyclone developed, while cooling due to sublimation, evaporation, and melting modified the PV much less strongly. The results of this case study show that the collocation of strong deposition heating with positive absolute vorticity in the upper troposphere can lead to substantial PV modification and a very different cyclone evolution to that when deposition heating is suppressed.