Assessing the risk of unprecedented flooding with event-based storylines

Hydrometeorological extremescause widespread socio-economic disruption and damage worldwide. Assessing the probability and impact of such events is critical for improving risk preparedness and emergency management. Ensemble climate simulations allow to extend the sample size of plausible weather extremesand estimate the probability of disruptive eventsthat may be unprecedented in the historical record. This approach, commonly known asUNSEEN (UNprecedented Simulated Extremes using ENsembles) has demonstrated a comparative advantage over traditional approaches that rely solely on observations for risk assessment. In this work, we apply the UNSEEN methodology to estimate the probability of river flooding in European medium-size catchments, using ensemble simulations of seasonal forecasts provided by the European Flood Awareness System (EFAS). Simulated values of river streamflow during rare events can be used as input for risk models to yield estimates of the socio-economic loss resulting from unprecedented floods. This framework promotes the co-design of flood risk storylinesthat can assist stakeholders and policy-planners in the development of mitigation and adaptation strategies to manage disasters.