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Biases and Tendencies
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Systematic model biases refer to forecast errors which occur routinely in
the same location and/or time. For example, 1000 mb forecast heights are
typically too low over the Philippine Sea. Systematic errors usually grow
in amplitude through the forecast period. The similarity to errors in the
model climatology over extended periods indicates these errors represent a
gradual drift from the climate of the atmosphere to that of the model. The
rate of model drift may vary from season to season, but the overall error
appears to be independent of the initial data.

Model tendencies refer to the similar handling of a synoptic forecast
situation without regard to location or season, and are usually related to
data density. For example, "Developing mid-latitude maritime lows tend to
be slightly underforecast and slow to deepen. Filling, mature lows are
overforecast and slow to fill. Deepening and filling lows show no
consistent directional or positional bias."