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The probabilistic QPF forecasts provide information about the Probabilities of exceeding a threshold show filled contour levels of probability that the 24-hour accumulation of precipitation will equal or exceed the given threshold. As an example, consider the 50mm/d threshold. If a point of interest falls within the 40% contour on the probability map, then the chance of precipitation exceeding 50mm is 40% or greater. As the threshold values increase, the probabilities of exceeding them decrease.
Probabilistic forecasts are computed based on the ensemble model forecasts. The ensemble forecasts provide uncertainty information about the QPF. This distribution is utilized to generate probabilistic forecasts of precipitation. A binormal probability distribution (density) function (PDF), which allows skewness, is constructed such that the mode is the QPF and the variance is that of the ensemble. The skewness is based on the position of the QPF in the ensemble distribution. This approach to estimating the three parameters for the binormal PDF is a variation on the method of moments.