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Land surface characteristics (soil moisture, leaf area index) have an impact upon temperature forecasts.    Significant differences in temperature can occur over a short distance where there is a sharp change of surface characteristics.   This can influence the location and development of subsequent convection.

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The right panel shows that on this occasion the magnitude of the dew point errors was even larger overall.   Again there are many possible reasons, but one candidate would be mishandling of moisture fluxes to/from the surface.  In turn these could relate to soil moisture errors, or errors in handling the biology of evapotranspiration.  

An influx of moist low-level air might also occur locally (e.g. effects of a strong sea breeze).  This can influence the location and development of subsequent convection.

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