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• tends to focus its perturbations above the boundary layer.  Uncertainties in weather parameters near the surface (e.g. temperature, visibility) may not be well represented.   The ensemble spread for such parameters may be too small.
  
• perturbations do not explicitly depend on the current synoptic pattern.  Occasionally the ensemble may show a very small risk of extreme weather beyond what is synoptically reasonable .  For example: convective heating from the seas in winter-time cold NW'ly outbreaks may be damped so that air-masses look unrealistically cold in one or two members(e.g. winter maritime convective heating may be dampened and some members may be unrealistically cold).

It should be stressed that overall stochastic perturbations undoubtedly do deliver clear improvements in the ensemble performance.

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