David Paul's Chance of Rain Definition Wrong?

[jasons2k]

This morning, David Paul called into the Roula and Ryan show on KRBE. They do this segment for listeners to "ask stupid questions".

One of the questions was "If you forecast a 30% chance of rain, does that mean you have a 30% chance of seeing rain or that 30% of the area will get rain".

This is a common question, but I was suprised at David Paul's response.

He said "It's the second answer and not the first. It means that 30% of the viewing area will see measurable precipitation in that 12-hour period".

Now, correct me if I am wrong, but I always always was taught the definition meant that "any given point in the forecast area has a 30% chance of measurable precip. within the forecast period".

Did he just give the wrong definition out to half a million radio listeners or am I crazy? About.com supports what I have always thought:

http://weather.about.com/od/c/g/chance_of_rain.htm

[srainhoutx]

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) 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.

http://www.srh.noaa.gov/ffc/?n=pop

[tireman4]

Jason, do you mean David Paul, the meteorologist and not Ron Paul, the politician..or maybe I am wrong.

[jasons2k]

Jason, do you mean David Paul, the meteorologist and not Ron Paul, the politician..or maybe I am wrong.

Oops - I have fixed it, thanks!

[jasons2k]

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.

http://www.srh.noaa.gov/ffc/?n=pop

Exactly. Just as I had thought. Somebody needs to tell David as he just gave the wrong definition to an awful lot of Houstonians this morning.

[TexasMetBlake]

I'd also like to add that chance of rain does not equal quantity. A lot of times during the summer (well not this summer, but you know what I mean) forecasters will put a 20% chance of rain in the forecast because of substantial heating in the boundary layer, seabreeze interaction, weak disturbances and boundaries that traverse the area are often able to squeeze SOMETHING out of the atmosphere. Since the atmosphere is fluid and very dynamical, it is nearly impossible to accurately model. Anyway, getting back to my point, I have driven down the road with family and sprinkles started hitting our window and the first thing they'd say is ''they didn't say it was going to rain today!" Just the opposite is true too---the weather guy has put 40% to 60% chance of rain in the forecast and if it doesn't come a downpour, then we were wrong. My family thinks if they don't get 2 inches of rain, then the forecast didn't verify. Ignorance. Chance of rain does NOT equal quantity. If it did nothing more than sprinkle, the weather guy was still right.

[Ptarmigan]

I have seen 100% chance of rain and results in widespread light to moderate rain which happens in the winter.