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heading to the valley next week, weather looks bad

yesrodcire · · Unknown Hometown · Joined Oct 2014 · Points: 5
dylan grabowski wrote: I know none of us are weather forecasters (or maybe we are), but "small chances of rain almost never pan out" doesn't necessarily make me want to jump in my car and drive 7 hours down south.
Well then don't.... lol. I don't know what you want from us here?

There is a chance of rain either so risk it or don't, we cant hold your hand here...
Kent Richards · · Unknown Hometown · Joined Jan 2009 · Points: 81
Explaining "Probability of Precipitation"

From srh.noaa.gov/ffc/?n=pop

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.
yesrodcire · · Unknown Hometown · Joined Oct 2014 · Points: 5

What's the given probability that meteorologists ever get there forecasts correct? Must be nice to have a job where you can screw up and be wrong almost everyday and still get to keep your job....

Marc801 C · · Sandy, Utah · Joined Feb 2014 · Points: 65
yesrodcire wrote:What's the given probability that meteorologists ever get there forecasts correct? Must be nice to have a job where you can screw up and be wrong almost everyday and still get to keep your job....
IIRC, if you say that tomorrow is going to be pretty much like today, you'll be correct about 74% of the time.
SRB25 · · Woodside, ca · Joined Nov 2014 · Points: 5
Aerili wrote: I used to think this was true. One time (when I lived much closer) I even made the 4-hour drive to the Valley going on that thought process. In reality the probability doesn't work like that. The 20% chance forecast turned into a nonstop downpour for over 24 hours. Granted, that is particularly unusual out West. But the "percent chance" says little about how much or how long you'll get the rain. Getting drenched in the Valley is a time honored tradition I'm pretty sure.
LOL. % chance is exactly that. A chance. Who doesn't have a story where the weather didn't cooperate. "I expected this rainy day to be rainier than this...That John Denver was full of shit man!"
dylan grabowski · · Denver · Joined Mar 2015 · Points: 95
yesrodcire wrote: Well then don't.... lol. I don't know what you want from us here? There is a chance of rain either so risk it or don't, we cant hold your hand here...
I got what I wanted. "20% chance of rain means 80% chance of sun." Making plans for the weekend trip.
JCM · · Unknown Hometown · Joined Jun 2008 · Points: 115
yesrodcire wrote:What's the given probability that meteorologists ever get there forecasts correct? Must be nice to have a job where you can screw up and be wrong almost everyday and still get to keep your job....
People love to make this joke, but honestly it isn't really true. There have been various studies comparing NWS forecasts with actual observed weather, and their forecasts track pretty well with observations. This means that when they say 20% chance of rain, in most places it does in fact rain about 20% of the times that make that prediction. This is where it trips people up. You'll see a 20%POP and think "It's going to be dry!" and make climbing plans...but 1 out of 5 times you will get rained on. You will probably remember the 1 time you got rained on more than the 5 times it didn't. Hence the "forecasters are wrong" sentiment. They weren't wrong; they actually nailed the forecasts (statistically). The issue is just that many of the consumers of the forecast don't understand basic probability.

I think that the remarkable thing is not how often the forecast is wrong, but in a long term averaged sense how accurate it is. The atmosphere is incredibly complex and chaotic, and the data available for making forecasts is actually pretty spotty and sparse. The fast that forecasts are possible at all is a wonderful thing.

Also, many people look at bad forecasts. Your local TV station, and even weather.com, statistically do worse than NWS. Nate Silver has an fantastic chapter on this in his book. www.noaa.gov is the way.

Aerili wrote: I used to think this was true. One time (when I lived much closer) I even made the 4-hour drive to the Valley going on that thought process. In reality the probability doesn't work like that. The 20% chance forecast turned into a nonstop downpour for over 24 hours.
Making plans based on almost any forecast (except for the rare 0%POP) is a bit of a gamble. A 10% to 20% POP is a pretty safe gamble, but occasionally you will get skunked. That is a risk you have to take. If you waited for a perfect forecast every time, you'd never get any climbing done (especially in the PNW).

dylan grabowski wrote: I got what I wanted. "20% chance of rain means 80% chance of sun." Making plans for the weekend trip.
Glad I could offer some PNW perspective. I swear those CA and AZ climbers see a cloud and freak out.
Jon Ruland · · Tucson, AZ · Joined May 2007 · Points: 646
JCM wrote:The atmosphere is incredibly complex and chaotic, and the data available for making forecasts is actually pretty spotty and sparse. The fast that forecasts are possible at all is a wonderful thing.
yeah that's been my thought as well. predicting multiple days in advance has got to be especially difficult.

JCM wrote:Nate Silver has an fantastic chapter on this in his book.
he has a book? what's it called?

JCM wrote:I swear those CA and AZ climbers see a cloud and freak out.
accurate. a couple weeks ago i woke up to see not one, not two, but THREE clouds over mount lemmon and i almost cancelled my climbing plans for the day. but i went up there anyway and we managed to escape with our lives.
JCM · · Unknown Hometown · Joined Jun 2008 · Points: 115
Jon Ruland wrote: he has a book? what's it called?
The Signal and the Noise: Why So Many Predictions Fail--but Some Don't

amazon.com/The-Signal-Noise…

Highly recommended.
yesrodcire · · Unknown Hometown · Joined Oct 2014 · Points: 5
JCM wrote: People love to make this joke, but honestly it isn't really true. There have been various studies comparing NWS forecasts with actual observed weather, and their forecasts track pretty well with observations. This means that when they say 20% chance of rain, in most places it does in fact rain about 20% of the times that make that prediction. This is where it trips people up. You'll see a 20%POP and think "It's going to be dry!" and make climbing plans...but 1 out of 5 times you will get rained on. You will probably remember the 1 time you got rained on more than the 5 times it didn't. Hence the "forecasters are wrong" sentiment. They weren't wrong; they actually nailed the forecasts (statistically). The issue is just that many of the consumers of the forecast don't understand basic probability. I think that the remarkable thing is not how often the forecast is wrong, but in a long term averaged sense how accurate it is. The atmosphere is incredibly complex and chaotic, and the data available for making forecasts is actually pretty spotty and sparse. The fast that forecasts are possible at all is a wonderful thing. Also, many people look at bad forecasts. Your local TV station, and even weather.com, statistically do worse than NWS. Nate Silver has an fantastic chapter on this in his book. www.noaa.gov is the way. Making plans based on almost any forecast (except for the rare 0%POP) is a bit of a gamble. A 10% to 20% POP is a pretty safe gamble, but occasionally you will get skunked. That is a risk you have to take. If you waited for a perfect forecast every time, you'd never get any climbing done (especially in the PNW). Glad I could offer some PNW perspective. I swear those CA and AZ climbers see a cloud and freak out.
Lol but it is true! Watch the nightly news and listen to their 7 day forecast, check again a week later and how correct were they? I would guess less than 30% accuracy.... So they are putting out forecasts that are wrong 70% of the time, that's a majority! What other job can you be wrong most of the time and still have a job?

I will admit 2-3 days out the meteorologist are pretty accurate, but the fact is anymore than 3+ days out and their accuracy is basically down to zero. And even I can usually judge what the weather is going to do tomorrow based off of current observations and radar imagery so meteorologists are nailing the 2.5 day forecast window or what? So again they aren't able to accurately predict weather much further out than the layperson and are continually wrong... must be a nice job!
Aerili · · Los Alamos, NM · Joined Mar 2007 · Points: 1,875
SRB25 wrote: LOL. % chance is exactly that. A chance. Who doesn't have a story where the weather didn't cooperate.
Thanks for letting me know the science is so simple.

JCM wrote:Making plans based on almost any forecast (except for the rare 0%POP) is a bit of a gamble. A 10% to 20% POP is a pretty safe gamble, but occasionally you will get skunked. That is a risk you have to take. If you waited for a perfect forecast every time, you'd never get any climbing done (especially in the PNW).
I had no idea.
mucci · · sf ca · Joined Jan 2007 · Points: 655

I would have never accomplished anything if I planned my Yos trips around the weather forecast.

Don't even flinch until it hits 40%

I say call the bluff.

Kevin DeWeese · · @failfalling - Oakland, Ca · Joined Jan 2007 · Points: 981
mucci wrote:I would have never accomplished anything if I planned my Yos trips around the weather forecast. Don't even flinch until it hits 40% I say call the bluff.
It's the fire reports that get you every time!
Jon Ruland · · Tucson, AZ · Joined May 2007 · Points: 646
mucci wrote:I would have never accomplished anything if I planned my Yos trips around the weather forecast. Don't even flinch until it hits 40% I say call the bluff.
ok mother nature, i ain't blinkin. your move.
SRB25 · · Woodside, ca · Joined Nov 2014 · Points: 5
Aerili wrote: Thanks for letting me know the science is so simple. I had no idea.
Don't mention it!
SRB25 · · Woodside, ca · Joined Nov 2014 · Points: 5
JCM wrote: The atmosphere is incredibly complex and chaotic...
The atmosphere has layers and shit!
Kevin DeWeese · · @failfalling - Oakland, Ca · Joined Jan 2007 · Points: 981
SRB25 wrote: The atmosphere has layers and shit!
I C wut U did
Guideline #1: Don't be a jerk.

Northern California
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