In trying to produce the best possible NWP forecast, you might think that the model initial analysis should match exactly the observations used in that analysis, but you would be wrong.

Let's explore what's behind this counter-intuitive misconception.

The initial analysis comes from a complicated combination of the observations and a short range model forecast called the trial field (also referred to as the first guess field), and is designed to provide the best possible starting point for the forecast model. The analysis must account for such factors as differing accuracy of the various observing systems, the possibility of incorrect observations, and the relative importance of the trial field and the observations. The analysis must also be consistent with the model's own resolution and its own physics. For these reasons, the model initial analysis will differ somewhat from the observations.