The UK sits at the downstream end of the North Atlantic (NA) storm track, meaning that the passage of extratropical cyclones represents a considerable natural hazard. Some of the most severe hydrological impacts are experienced when intense vapour fluxes in a cyclone’s warm sector encounter the uplands of Britain, driving intense orographic rainfall. The frequency of such ‘Atmospheric Rivers’ (ARs) is anticipated to increase as the climate continues to warm, due to absolute humidity rising in line with the Clausius Clapeyron relation. It is against this backdrop that Storm Desmond (Dec, 2015) broke long-standing rainfall and runoff records across the UK, in a year that was (at the time) the warmest on record. In this research, we ask to what extent the hydrologically-intense nature of Desmond can be attributed to climate change. Using a novel observation-based method, we assess the processes contributing to Desmond’s severity, and find that the likelihood of such an intense AR has increased by more than a factor of two since preindustrial times. This research highlights (i) the complementary role observational, process-based studies can play in climate change attribution studies; and (ii) the important influence of NA sea surface temperatures on UK hydroclimatic extremes.