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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 verifying at the same time.
Fig4.1.3: A sequence of 1000hPa Mean Sea Level Pressure forecast charts ranging from T+156h to T+96h, all verify at 00UTC 20 August 201024 October 2022. The forecast details differ between the forecasts but large-scale systems (a low near Ireland, a high over central Europe, a trough over towards the southern Baltic States) are common features. The T+144hr forecast from 14 August predicted a southwesterly gale over the British Isles six days later156hr 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+132hr 144hr and T+120hr 132hr runs showed a weaker circulation. The same cautious approach would have minimized the forecast “jump” with the arrival of the T+108hr forecast.
Small-scale details can be filtered out to highlight the predictable scale. This does not necessarily have to be done subjectively. Retaining only the first 20 spectral components filters away all scales smaller than1000km and brings out the more predictable large-scale pattern (see Fig4.1.4).
Fig4.1.4: Same as Fig4.1.3 but based on just the 20 largest spectral components. Five of the six forecasts now show much larger coherence, with a cyclonic feature approaching the British Isles and a stationary high pressure system over central Europe.
A synoptic example of combining EM and probabilities
However, spectral filtering does not take into account how the predictability varies due its flow dependency; a small-scale feature near Portugal might be less predictable than an equally sized feature over Finland. To avoid over-interpreting the EM, in particular underestimating the risk of extreme weather events, it should preferably be presented together with a measure of the ensemble spread or event probabilities; these will convey an impression complementary to the EM. Since the EM and the probabilities relate naturally to each other, they should be presented together. So, for example, the EM of the MSLP (or 1000hPa) presented together with gale probabilities will put the latter into a synoptic context that will facilitate interpretation (Fig4.1.5).
Fig4.1.5: 1000 hPa forecast from 13 August 12 UTC +156 h to 16 August 00 UTC + 96 h, all valid at 20 August 00 UTC. Full lines are the 1000 hPa geopotential EM overlaid 4: A sequence of Mean Sea Level Pressure forecast charts overlaid by the probabilities of wind speeds > 10 >10 m /s. Probabilities are coloured in 20% intervals starting from 20%. s-1 ranging from T+156h to T+96h, all verify at 00UTC 24 October 2022. Probabilities are coloured according to the scale. Compare with Fig4.1.3 and Fig4.1.4.
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The ensemble assesses the forecasts in a Fig4.1.3 and Fig4.1.4 showed an example of how filtering less predictable synoptic scales can increase the accuracy and reduce the jumpiness of forecasts. This filtering is much better accomplished through the ensemble in Fig4.1.5. The ENS treats the same synoptic situation in a more consistent and optimal way as its flow dependency serves as a superior dynamic filter. It gives the probability of an outcome (in this case strength of wind) rather than relying on an individual solution.
The T+12 h ensemble forecast is used here as an analysis proxy for the verification of the above forecasts (see Fig4.1.6).
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