Probabilistic forecasting

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Probabilistic forecasting is a technique for weather forecasting which relies on different methods to establish an event occurrence/magnitude probability. This differs substantially from giving a definite information on the occurrence/magnitude (or not) of the same event, technique used in deterministic forecasting. Both techniques try to predict events but information on the uncertainty of the prediction is only present in the probabilistic forecast.

The probability information is typically derived by using several numerical model runs, with slightly varying initial conditions. This technique is usually referred to as Ensemble forecasting or EPS (ensemble prediction system).

According to how many members (i.e. individual model runs) of the ensemble predict the same event with the same magnitude, one can compute the final probability of the given event. Canada has been one of the first countries to broadcast their probabilistic forecast by giving chances of precipitation in percentages.