This may be a dumb question but what is the definition of the term POPS that I keep seeing on the forcast discussion on the national weather service sites? Its proberly something simple but I cant come up with it.
Precipitation Probabilities (PoP): It is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is defined as equal to or greater than .01 inch or .2 mm. Normally, the period of time is 12 hours, unless specified otherwise. The forecast area, or zone, is generally considered to be a county. In some geographically unique areas (mountains), the forecast area/zone may consist of portions of a county or two counties. At times, some NWS forecasters will use occasional or periods of to describe a precipitation event that has a high probability of occurrence, i.e., they expect any given location in a forecast zone area to most likely have precipitation, but it will be of an on and off nature. Usually, away from the mountains, each and every county is a forecast zone area itself. The following table shows precipitation probabilities used in National Weather Service forecasts and a brief explanation of each.
Precipitation Probabilities (PoP) used in National Weather Service Forecasts
Expressions of Uncertainty
Equivalent Areal Qualifiers (convective only)
10 percent none used isolated, or few
20 percent slight chance widely scattered
30-40-50 percent chance scattered
60-70 percent likely numerous (or none used)
80-90-100 percent (categorical) none used none used
Source: National Weather Service