A case study of winter upward lightning is analysed. In particular, the study focuses on the meteorological aspects that favoured the triggering of upward lightning by an instrumented tower in the eastern Pyrenees. Starting from the lightning currents measured at the Eagle Nest Tower, and taking advantage of other singular meteorological instruments deployed in the region, the obtained results showed how a small winter thunderstorm can create the necessary conditions for the inception of self-initiated upward lightning from a small tower. Beyond the particularities of the present case study, the results provide new evidence on the necessity to include upward flashes in risk assessment. © 2018 IEEE.