MedEWSa - From hazard and impact indicators to threshold and risk matrix

This MedEWSa slide deck explains how it is possible to use impact information to make decisions, explaining why evidence does not automatically produce action. Decisionmakers must weight false alarms, missed events, resources and confidence. It sets out the chain from indicator to threshold, trigger and action. It shows how effective triggers balance reliability, lead time, consequences and what can practically be done, and how they change as an event approaches. Every decision comes down to two questions, how likely and how impactful, which the risk matrix brings together, following WMO guidance. A worked example of the CEMS GloFAS rapid impact assessment shows step by step how flood forecasts become categories of affected population. Successive forecasts for an August 2023 event then show how the decision is updated over time. The slide deck ends by stressing that risk categories support proportionate action but do not replace official mandates, local knowledge or expert judgement.