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 UNDERSTANDING POP (CHANCE OF PRECIPITATION)

METEOROLOGIST JEFF HABY

POP is the probability of any particular point location within a forecast area receiving measurable precipitation in a given time period. POP is determined by two parameters: The probability that any precipitation will occur in forecast area AND predicted areal coverage of precipitation if precipitation does occur. Thus, areal coverage is just one aspect of POP. The chance that any measurable precipitation will occur in the first place within the forecast area must also be considered. When referring to POP it is most accurate to say, "There is a ___% POP that any particular measurement station in the viewing area will get precipitation". For example, when averaged over many 30% POP days, a particular station should have precipitation 30% of the time if the forecaster or computer model is accurate.

Suppose during the course of a year that a Dallas measurement station has a 30% POP on 100 individual days. If the POP prediction is fairly accurate over the long term, it should have precipitated at the Dallas station on about 30 of those days. Thus, think of POP in terms of a long term average prediction. All to often, people assume a 20% chance of rain means 20% of forecast area will get rain on that one day. This is often not the case for any one day. Often no precipitation occurs at all in the forecast area on days with a 20% POP since one component of POP is the chance that any precipitation will develop.

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