Breakdown: What 40% chance of rain means & why it’s confusing

Breakdown: What 40% chance of rain means & why it's confusing

MEMPHIS, Tenn. (WMC) - Meteorologists talk about the probability of percentage all the time, but many people do not understand what that rain percentage actually means.

Most people assume that a 40 percent chance of rain means it is going to rain 40% of the day, but that thought process is not correct.

According to the National Weather Service, Probability of Precipitation or (PoP) is “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.”

The WMC Action News 5 First Alert Weather team covers 27 counties in three states, so the PoP is actually describing the chance of rain at any point over our entire viewing area. This is confusing because the 40 percent (or any other percentage we forecast) is not for one particular area.

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 in 80% of the area, the PoP (chance of rain) is 40%. (PoP = .5 x .8 which equals .4 or 40%.)

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