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In general, temperatures are forecast fairly well over the globe.  On average, systematic errors in forecast 2m temperatures are generally <0.5°C.  Biases in 2m temperature (verified over land) vary geographically, as well as with season, time of day and altitude.  Larger biases and errors occur over orography or in snow covered areas.  

Diurnal temperature changes are strongly influenced by incoming and outgoing heat flux which itself is governed by the extent and thickness of cloud cover.  Uncertainty in the model analysis of cloud can have a strong impact on forecast errors.  

Most  Most of the large errors seem to occur when the surface temperature is very cold, and the lowest levels may levels may become extremely stable.   In such very stable air tiny amounts of energy can correspond to large temperature changes at the surface because surface because there is no convection to mix energy through the lower atmosphere.  This is the main physical reason for large errors being relatively commonplace in such circumstances.  Temperature errors often don’t depend strongly on the forecast range.

The near-surface inversion is likely to be most influential and errors more likely with high pressure and calm conditions.  It is vital to compare the observed and forecast thickness and extent of low cloud and the temperature and humidity structure of the lowest atmosphere.  

Effects contributing to temperature errors

Near-surface temperatures are related to a variety of processes:

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The forecast vertical profiles show that the model was fairly good at representing temperature inversions.  However, the model locally missed clouds and fog by a small margins.  Such small errors had a large impact on surface temperatures.    The warm area in central northern Spain associated with high ground above the overnight inversion.

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