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 In the analysis below (Fig6.4.1 and Fig6.4.2), forecasts by all IFS models progressively correlate less well with observed values as forecast lead-time increases.  In these graphs HRES has 9Km resolution, the medium range ensemble has 18Km resolution.  The unperturbed ensemble control (CTRL) performs better than any individual perturbed member of ensemble (PF in the diagrams).  The ensemble mean smooths out many smaller scale features and performs best against the verifying analysis.  This supports preferential use of the ensemble mean in practical forecasting.  Typically ACC falls to 0.6 at around day8 or day9 for ensemble control (CTRL), and at around day10 for the ensemble mean (EM).

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 Fig6.4.3:  Time series of the annual running mean of anomaly correlations of HRES 500 hPa height forecasts evaluated against the operational analyses for the period 2000 until 2022.  Values are running 12month average scores.  Forecast lead-times in days ahead - 3(blue), 5(red), 7(green) and 10(yellow) - are shown for scores averaged over the northern extra-tropics (bold lines) and southern extra-tropics (thin lines).  The shading shows differences in scores between the two hemispheres at the forecast ranges indicated.   Currently 3-day, 5-day, 7-day, 10-day forecasts have attained approximately 98.5%, 92%, 80%, 50% anomaly correlation (ACC) respectively.

Current Anomaly Correlation Coefficient (ACC) scores for northern hemisphere, Southern Hemisphere, tropics and Europe are available.

The score for the northern hemisphere extra-tropics is a primary headline score of the ECMWF high resolution forecast (HRES). 

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Fig6.4.4: The plot shows the forecast lead-time (in days) at which Anomaly Correlation Coefficient (ACC) of the HRES forecast dropped below 80% for:

• the month mean (blue line, with blue spots at each month),
• twelve-month mean centred on that month (red line).

Verification follows updated WMO/CBS guidelines as specified in the Manual on the GDPFS, Volume 1, Part II, Attachment II.7, Table F, (2010 Edition - Updated in 2012).