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The philosophy of ensemble forecasting is to capture possible outcomes rather than a single definitive answer. Thus some variability in a sequence of NWP model forecasts is to be expected. Every new forecast run is, on average, better than the previous one - but it is also different. New observations modify previous analyses of the atmospheric state, and therefore . Therefore forecasts generated from these new analyses evolve and develop differently to earlier results. Usually , the differences in the forecasts are no more than moderate, but occasionally , differences can be quite large. These may appear as a significant forecast jump in evolution of the atmosphere (eg e.g. developing a more mobile or amplified upper trough or a significant increase in modelled cloud cover). This has a consequent effect on forecast values.
This uncertainty This is an unavoidable consequence of a an imperfect dynamical forecast system - but it's not a problem as such. To the uninformed user or customer , this can be very disconcerting and cause a lack of confidence. To the informed forecaster it is an opportunity to deduce additional information and produce a better probabilistic forecast which is arguably of more use to the customer.
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