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What Does a "20% Chance of Rain" Actually Mean?

Weather Talk

POSTED: Thursday, July 24, 2014 - 12:18pm

UPDATED: Friday, July 25, 2014 - 4:15pm

We all see and hear about a percentage chance of rain in the forecast, but very few of us actually know what a 20% chance means.

We have all have wondered at some point what does “20% chance of rain" mean? Does it mean it will rain 20% of the day? Does it mean it shall rain over 20% of the area?Does it mean 20% of the forecasters think it is going to rain? Is it like Vegas, the odds are 20% that we will see some rain?

In today’s “Weather Talk”, we will find the answer to what rain percentages actually mean.

Many of us do not quite understand when the National Weather Service or TV Forecaster talks about the “chances of rain.” When there is a “chance” the NWS is referring to a “measurable amount of rain” they are talking about at least 0.01” or 1/100th of an inch. The National Weather Service (NWS) normally includes the wording "chance of rain" or "chance of precipitation". This is called a "PoP" (probability of precipitation) statement:

Here is an example taken from yesterday’s Zone forecast for the Gila Wilderness:
NMZ402-403-408-232215-
SOUTHERN GILA HIGHLANDS/BLACK RANGE-
SOUTHERN GILA FOOTHILLS/MIMBRES VALLEY-
EASTERN BLACK RANGE FOOTHILLS-
INCLUDING THE CITIES OF...SILVER CITY...LAKE ROBERTS...KINGSTON...
FORT BAYARD...MIMBRES...FAYWOOD...HURLEY...GRANT COUNTY AIRPORT...
HILLSBORO...WINSTON
326 AM MDT WED JUL 23 2014

TODAY...PARTLY CLOUDY WITH ISOLATED SHOWERS AND THUNDERSTORMS IN
THE MORNING...THEN MOSTLY CLOUDY WITH SCATTERED SHOWERS AND
THUNDERSTORMS IN THE AFTERNOON. HIGHS IN THE UPPER 80S. SOUTHEAST
WINDS 10 TO 15 MPH. CHANCE OF PRECIPITATION 40 PERCENT.

The National Oceanic and Atmospheric Agency (NOAA) National Weather Service (NWS) have come up with a relatively simple formula to explain rain percentages. I know you did not want to do any math, but this is really straight forward. Here is how the NWS presents the formula:

“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 measurable precipitation, if it occurs at all.”

 Simply put:

CHANCE OF RAIN = AREA COVERAGE x FORECASTER CONFIDENCE

So in my ”Week’s Worth of Weather” forecast for tonight I say there is there is a 20% chance of rain for Monday. I am 100% confidence that 20% of our area will see measurable rain of 0.01” or more.

20% Area Coverage x 100% Forecaster Confidence = 20%

What if I think that 20% of our area will have rain but I’m only 50% confident in that forecast?

20% Area Coverage x 50% Forecaster Confidence = 10%

Sometimes the computer prediction models are not in agreement. Also many times the models say one thing as the environment say another. This introduces some doubt; therefore I am or the forecaster is not as confident about the possibility of rain.

This formula does not factor in the amount of rain an area might receive or the rainfall intensity. The rain rate may be anywhere from a few sprinkles to a heavy downpour! The “rain chance” just means that some  given point or official rain gauge  in the forecast area is going to see 0.01” or more of rainfall.

Now if you stay in one place your chance of rain is 20%. You increase your odds if your travel around the forecast area. We go to work, drive to the mall, and go to school. Most people rarely stay in one place all day! The more you travel around in the forecast area, the greater the odds of you seeing some rain.

Some TV forecasters now choose not to use “rain percentages” in the forecast. They just have a rain or thunderstorm icon and with text or they verbally mention if it will be a stray shower or rain over most of the area. That works too. I and some other forecasters use the chance of rain as the odds of seeing rain during certain day parts at any given point on the map. The confidence factor really does play into the percentage, the high the number the more confident we are it is going to rain. So the next time you see a 20%, chance of rain in a forecast you’ll know better what that percentage represents. Just know a 40% probability or above means you better have a “Plan B” if have an outdoor event or fun scheduled for that day.

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX
cdebroder@ktsm.com
www.facebook.com/pages/Charles-DeBroder/
https://twitter.com/wxchuckNC9
www.ktsm.com/weather

 

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