One of the most promising ways of de-carbonising the energy sector is through increasing the amounts of inherently uncertain variable renewable energy (VRE) generation in power systems.Typically, stochastic energy system studies have focused solely on the day-ahead horizon of 36 hours ahead of time, while studies about hydro-thermal scheduling and expansion planning often neglect VRE uncertainty entirely. In this work, the potential benefits of extending the horizon of VRE forecasts on the operation of hydro-dominated power systems was studied using a future Nordic system case study. 15-day ensemble weather forecasts were processed into realistic VRE forecasts up to 348 hours ahead of time, and their impacts on power system operations were simulated using stochastic unit commitment and economic dispatch optimisation. Increasing the length of the modelled forecast horizon reduced the total yearly operational costs of the system by 0.18–0.41%, as well as the spillage of run-of-river hydropower and the curtailment of wind power by 0.42–0.47 and 0.05–0.07% points respectively.