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The figures above show cyclones in the ensemble (dots), where the colour indicates the strength of the maximum wind speed at 1 km height connected to the feature. All the plots are valid 28 October 00 or 12 UTC. The contour shows the MSLP for the control forecast. The forecasts are from 0UTC runs starting from 21 Oct to 28 Oct. With shorter lead times, the spread of the features decreases. We also see that the centre of gravity for the red-orange dots moves eastward with decreasing lead time, illustrating the too westward development in the model.
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The spread in the 00Z Sunday T+36h forecasts, reproduced below as a single frame (left), is especially striking, showing an ordinary but active autumnal frontal wave (green dots = 55-60kts) in a few members, through to a once-in-a-lifetime intense (red = 75-80kts) low in rather more members. Example snapshots from each of these two categories are shown to the right. This is a 'nightmare scenario' for forecasters, but at the same time probably gives a true reflection of uncertainty in what appears to have been a dynamically very sensitive situation. It would seem, perhaps, that uncertainty at other times was under-estimated?? The intensity range (as inferred from colour range) in forecasts from slightly earlier times, on the dalmatian animations above, seems, curiously, to have been less.
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CDF for wind gusts (left) and maximum mean wind speed (right) for Reading valid on the 28 October. Different colour represents different initial times and the o-suite (38r2, solid) and e-suite (40r1, dotted). Comparing different initial times, the forecast from 26 October (green and black), were the most severe. For the forecasts from the 27 October the wind speed was less. This is partly because the development of the cyclone occurred later and therefore had not reached such a deep stage when it passed over Reading. Comparing e-suite and o-suite, we see that the winds (both mean and gusts) are less in the e-suite and o-suite.
With regard to ensemble spread, from a well-behaved ensemble, one would expect that, between consecutive runs, the cdf curves tend to become steeper, and gradually move less and less in the lateral direction as the event approaches. There are undoubtedly complicating factors for wind strength when one is close to the low track (which can for example give bimodal distributions = cdf jumps) but if one puts that aspect to one side it would seem that these EPS runs, in E and O-suites, are not behaving quite as they should. The inter-quartile range, which can be exactly inferred from these plots (and likewise the standard deviation which can be usefully estimated) appear not to be reducing with lead-time, and there are also notable lateral jumps in the profiles at shorter leads. A characteristic of a well-behaved ensemble is that measures of spread, for a given validity time, will reduce as the lead time reduces, i.e. for later and later forecasts.
CDF for wind gusts (left) and maximum mean wind speed (right) for Reading valid on the 28 October. Different colour represents different initial times and the o-suite (38r2, solid) and e-suite (40r1, dotted). Comparing different initial times, the forecast from 26 October (green and black), were the most severe. For the forecasts from the 27 October the wind speed was less. This is partly because the development of the cyclone occurred later and therefore had not reached such a deep stage when it passed over Reading. Comparing e-suite and o-suite, we see that the winds (both mean and gusts) are less in the e-suite and o-suite.
4.3 Monthly forecasts
We cannot expect to see a strong signal for extreme cyclones in the monthly forecast. However, we can investigate whether the environment was favourable for windy conditions. The figures above show the weekly MSLP anomaly for the week starting on the 28 October. The first figure shows the forecast from the 28 October (Monday of the verifying week), followed by 24 Oct, 21 Oct, 17 Oct, 14 Oct and 10 Oct. At least for the 5 first forecasts a positive NAO signal (negative MSLP anomaly in the northern Atlantic and positive further south) is present. A positive NAO is usually leading to stronger winds over western Europe.
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