An interpolation function is provided to automatically create grids intermediate in time between two forecast grids which were prepared from guidance or by a forecaster. Interpolation is used when conditions are expected to change gradually between completed grids. Interpolation assists preparation of forecasts, because it reduces the demand on the forecaster to draw or edit all required weather elements at all required times, when changes are needed in guidance. If the forecaster judges that some weather element cannot be estimated correctly by interpolation between completed grids, then at least some of the intermediate grids must be prepared by hand. 2.1 Interpolation of stationary changes Stationary changes are gradual changes in a continuous weather element.
Examples are diurnal changes in temperature, dewpoint, wind, and pressure.
Interpolating grids by stationary changes is done by linear interpolation at each grid point from known values at the same grid point immediately preceding and following the time, or by polynomial interpolation at each grid point from several known values preceding and following the time of the missing data value. Stationary interpolation treats each grid point separately and does not consider what changes occur at other points in the grid. The values at a grid point are treated as samples of a continuous variable at different times. Interpolated values are derived from known values and times. The process is automatic and requires no forecaster control or input.