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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 Fig4.1.3 shows 1000hPa forecasts from six sequential runs of HRES  (identical to CONTROL-10) verifying at the same time. 

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”“jump”, when the subsequent T+144 and T+132 runs showed a weaker circulation.  The same cautious approach would have minimized the forecast “jump” “jump” with the arrival of the T+108 forecast.

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Over-interpreting the ensemble forecast or underestimation of the risk of extreme weather events should be avoided.  This can be helped if ensemble forecasts are presented together with a measure of the ensemble spread or event probabilities.   As an example, gales are put into a synoptic context if ensemble forecasts of MSLP (or 1000hPa) are presented together with gale probabilities (Fig4.1.5).

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 The T+12 ensemble forecast is used here as an analysis proxy for the verification of the above forecasts (see Fig4.1.6).

 Fig4.1.6: 1000 hPa EM ensemble mean DT 19 August 12 UTC T+12 h valid at VD 20 August 00 UTC may serve as a proxy analysis for verification because of the small forecast range and the fact that the EM, thanks to the initial anti-symmetric nature of the perturbations, is almost identical to the CTRL. The probabilities essentially show where the verifying wind speed was > 10 m/s.

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In order to estimate and compensate for any model drift the model output is compared with the corresponding model climates (M-climate for medium range, ER-M-climate for extended range, S-M-climate for Seasonal seasonal forecasting) for the current forecast date.  This is derived using the same model construction as the ensemble from a number of perturbed forecasts based on calendar dates surrounding the date of the current ensemble run using historical data from several years.  Systematic errors are then corrected during post-processing after the forecast is run. 

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