A forecast map visualises a model calculation for a specific time and variable. It is not a photograph of the future and does not have equal accuracy everywhere.
Before reading the colours, check four things: the model, the initialisation time, the valid time and the unit shown on the legend.
Initialisation and valid time
Initialisation is the time from which the simulation began. Valid time is the moment represented by the map. Their difference is the forecast lead time.
Maps valid at the same time but produced by successive runs show whether the solution is stable or changing.
Resolution and local detail
Grid resolution is not the same as real accuracy. A cell several kilometres wide smooths mountains, coasts and islands, while thunderstorms may develop between grid points.
Zoom enlarges the display, not the underlying information. The original resolution of both model and variable still matters.
Colours, contours and uncertainty
Always read the legend. The same red can mean high temperature, heavy rain or a large anomaly on different maps.
For important decisions, compare successive runs, more than one model and an ensemble where available. Greater disagreement means lower confidence in detail.
These explanations and calculations are educational. For safety, occupational, health, marine or aviation decisions, follow the appropriate official services.