The way NWP data is retrieved, interpolated and presented can affect the indicated value of the forecast parameters at a given location.   ECMWF forecast products can be retrieved:

• on a wide range of spatial resolutions, from regular and rotated latitude-longitude grids to the original resolution.
• on pressure, theta (isentropic) or potential vorticity (PV) levels, depending upon the parameter.

Forecast parameters are stored as spectral components but can be interpolated to any specified latitude-longitude grid.  This interpolation can also be applied to near-surface parameters which has a (cubic octahedral) grid as the native storage method.  

Direct use of these values, particular gridded values, is strongly recommended, especially for precipitation and other surface fluxes.  This avoids undesired effects of interpolation.

(FUG Associated with Cy49r1)