If you watch WDRB often, or look at forecasts regularly, you are probably familiar with some of the terminology we use to explain the weather. One example is: there is a 30% chance for showers and storms on Monday. That 30% chance is known as the "probability of precipitation". The "Probability of Precipitation" describes the chance of precipitation occurring at any point you select in the area. You may sometimes hear meteorologists abbreviate that term and mention the word "POP" or "POPS".
But what exactly does the 30% chance mean? I hear TONS of varying definitions of what people *think* it means. In my opinion, it is one of the most misunderstood terms we use everyday in our forecasts. Let's try to fix that!
A Probability of precipitation describes:
- The chance of precipitation...
- At a particular point...
- Over a certain period of time...
How do forecasters arrive at this value?
Mathematically, PoP is defined as follows: PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measurable precipitation, if it occurs at all.
So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur (confidence is 100%), he/she is expressing how much of the area will receive measurable rain. (PoP = "C" x "A" or "1" times ".3" which equals .3 or 30%.)
But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 60 percent of the area, the PoP (chance of rain) is 30%. (PoP = .5 x .6 which equals .3 or 30%.)
In either event, the correct way to interpret the forecast is: there is a 30 percent chance that rain will occur at any given point in the area.
