### Technical Definition of Precipitation % - Directly from the National Weather Service

EXAMPLE

ZONE FORECASTS FOR NORTH AND CENTRAL GEORGIA

NATIONAL WEATHER SERVICE PEACHTREE CITY GA

119 PM EDT THU MAY 8 2008

GAZ021-022-032034-044046-055-057-090815-

CHEROKEE-CLAYTON-COBB-DEKALB-FORSYTH-GWINNETT-HENRY-NORTH FULTON-

ROCKDALE-SOUTH FULTON-

INCLUDING THE CITIES OF...ATLANTA...CONYERS...DECATUR...

EAST POINT...LAWRENCEVILLE...MARIETTA

119 PM EDT THU MAY x 2008

.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF

SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR

STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH.

.TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND

THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS

AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST

WINDS 5 TO 15 MPH. **CHANCE OF RAIN 40 PERCENT.**

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at * any* point you select in the area.

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 measureable 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 ".4" which equals .4 or 40%.)

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

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.