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Beaches, barbecues and the algebra of rain

A "30 percent chance of showers," but what does that mean?

Like the Mister Softee jingle, the official forecast for tomorrow has a familiar ring: A "30 percent" chance of showers. The chances bump up to 50 or 60 percent Saturday night and Sunday throughout the region.

But what exactly does that mean? How helpful is that forecast to people planning outdoor parties or trips to the Shore?

In all probability, a 30 percent chance of showers doesn't mean quite what you think. Technically, it means there's a 30 percent chance to at least 0.01 inches of rain will fall at any given point in a forecast period during a given 12-hour period.

The National Oceanic and Atmopsheric Administration tells us that the precipitation probability, or POP,  is derived from a simple algebraic equation -- POP = C x A.

C represents the likelihood that precpitation will occur somehwere in the forecast area, and A is the percent of the area that will get whacked.

So if there's an 80 percent chance of measureable rain falling over 60 percent of the forecast area, the precipitation probability would be 48 percent, rounded to 50 in the official forecast.

Got that? Not that it will help with your planning