The grid values should not be considered as representing the weather conditions at the exact location of the grid point.  They should be considered as a time-space average within a two- or three-dimensional grid box.  The discrepancy between the forecast grid-point value and the verifying observed average value can be both systematic and non-systematic:

 

Fig3.2-1:  Comparison between NWP model output and observations ought ideally to follow a two-step procedure:


Fig3.2-2:  In reality, the comparison between NWP and observations must for simplicity bypass the area average stage.  This results in the systematic and non-systematic errors arising from distinctly different sources.  The effects related to the two green arrows in Fig3.2-1 are here combined into one.


Systematic errors maybe due to model deficiencies and/or observational representativeness.  These can be partly corrected by statistical means (e.g. model output statistics MOS).  A series of forecasts also helps with dealing with uncertainty.

Non-systematic synoptic errors can be dampened by different ensemble approaches (e.g. medium range ensemble, probability considerations, forecast error growth).

Sub-grid variability (notably for rainfall but other parameters too) can be addressed by downscaling.

Downscaling converts:

New downscaling techniques are being developed accordingly (see for example the Point Rainfall product).