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Probability of Precipitation: What Does it Really Mean?

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Probability of Precipitation, or "PoP," is a term that has been misused, abused and confused by meteorologists and the general public for decades.  Let;s clear up this little misunderstanding, once and for all.  By definition, the Probability of Precipitation (PoP), is the forecaster's best estimate of both the confidence in measurable precipitation (at least 0.01 inch) occurring AND the amount of areal coverage expected, if it does occur.  In mathematical terms, PoP = C X A, where C is confidence and A is areal coverage.  For example, if the forecaster is 80 percent confident in precipitation occurring over 50 percent of the area, that is PoP=(.80)(.50) = 40 percent chance of precipitation.  In lay terms, it is the chance of you getting wet where you are in the forecast area, during the time the forecast is valid. 

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