Forecasts issued by the National Weather Service routinely include a "PoP" (probability of precipitation) statement, which is often expressed as the "chance of rain" or "chance of precipitation".

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) simply describes the probability that the forecast grid/point in question will receive at least 0.01" of rain. So, in this example, there is a 40 percent probability for at least 0.01" of rain at the specific forecast point of interest!