The probabilities for a heat-wave are calculated based on 51 different forecasts (50 ensemble forecasts and 1 control run) with slightly different initial conditions for each point. Especially for longer lead times heat-wave probabilities of 30% and more indicate that there might be an upcoming heat-event.
The heat-wave probabilities displayed on the map are mean probabilities for a region. In regions with big differences in elevation or coastal regions the actual heat-wave probability can differ significantly within a region. The flyer: "How to use the medium range heat information tool" contains further information.
Please note that the medium-range heat information does not substitute national heat-warnings, but complements the national warning system with medium-range heat forecasts.