Often times you'll see a forecast something like this:
This Afternoon...Mostly cloudy with a 40% chance of showers and thunderstorms. Highs in the lower 80s.
The 40% is known as a (PoP) or probability of precipitation. This is a confusing term...even to a lot of meteorologists.
So...does this mean it will rain 40 percent of the time? Will it rain over 40 percent of the area? Well..kind of.
The official definition of a PoP describes the chance of precipitation at any given point in the forecast area.
So it's math time.
PoP = C x A where "C" is the confidence that precipitation will occur somewhere in the forecast area and where "A" is the percent of the area that will receive rain measurable precipitation, if it occurs at all.
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 80% of the area. The PoP is 40% (PoP = .5 x 8...which equals .4 or 40%.
So after all that...this is the correct way to interpret the forecast. There is a 40% percent chance that rain will occur at any given point in the area. But of course if it's raining where you are it's 100%.
Personally I'm not crazy about the (PoP). But then again it is nice to see it in the forecast pages and the extended. Because after all... when you see a chance of rain at 80...90..or 100% percent... you know chances are...you're gonna get wet.