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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