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The typical predictability is currently approximately twice the timescale, but might ultimately be three times the timescale.  Small baroclinic systems or fronts are currently well forecast to around Day2, cyclonic systems to around Day4 and the long planetary waves defining weather regimes to around Day8.  As models improve over time these limits are expected to advance further ahead of the data time.  Features that are coupled to the orography (e.g. lee-troughs), or to the underlying surface (e.g. heat lows), are rather less consistently well forecast.  The predictable scales also show the largest consistency from one run to the next.  Fig4.1-3 shows 1000hPa forecasts from six sequential runs of HRES HRES (identical to CTRL) verifying at the same time. 

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Fig4.1-3: A sequence of Mean Sea Level Pressure forecast charts ranging from T+156 to T+96, all verify at 00UTC 24 October 2022.  The forecast details differ between the forecasts but large-scale systems (a low near Ireland, a high over central Europe, a trough towards the southern Baltic) are common features.  The T+156 predicted gales over southern and northwest France.  It would have been unwise to make such a detailed interpretation of the forecast, considering the typical skill at that range.  Only a statement of windy, unsettled and cyclonic conditions would have been justified.  Such a cautious interpretation would have avoided any embarrassing forecast “jump”, when the subsequent T+144 and T+132 runs showed a weaker circulation.  The same cautious approach would have minimized the forecast “jump” with the arrival of the T+108 forecast.

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