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ENS Products - Dealing with Uncertainty

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


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