Natural events impact the power system infrastructure, causing multiple and dependent contingencies and even extensive disruptions. Given the high costs of these load disruptions, transmission system operators (TSOs) are interested in anticipating potentially critical scenarios and applying appropriate preventive and/or corrective countermeasures. In order to increase power system resilience against these events, a preliminary step is to analyze vulnerabilities to natural threats. The paper presents a general methodology and tool for resilience assessment, based on an extended definition of risk and suitable to support operation planning (identifying the most risky contingencies) and to perform engineering studies on what-if scenarios. The selected contingencies are analyzed, evaluating the system response based on various assumptions about threat intensity and component vulnerability. The application of the tool to a realistic case study, which refers to a real-life emergency scenario that occurred in a part of the Italian EHV / HV grid, shows the added value of the proposed approach in predicting critical conditions in power system operation compared to conventional security analyses.