JAM is a Met Office-led, multi-agency modelling capability which estimates, forecasts and provides advice on radiation emergencies from incidents within the UK and overseas. The JAM outputs are generated via an enhanced dispersion model and displayed as plume maps showing areas where thresholds for various impacts such as health, food restrictions and environmental contamination may be exceeded. The plume maps and accompanying expert narrative from across the agencies provide the Government's Scientific Advisory Group for Emergencies (SAGE) with a single, coherent output to support decision making.