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Message
Weather forecasts and statistics
Posted on 4/28/16 at 9:30 am
Posted on 4/28/16 at 9:30 am
I get that the weather man isn't going always be right. I'm not arguing that. What annoys me is when they put 0% chance of rain or 100% chance of rain. That means there is absolutely no chance, or that it absolutely will rain without any room for doubt. Which is fine if they are right, but often times you'll see 0% and then they change it the next day. Why do they do this???? Especially in the extended forecast.
I've got something on Saturday and the local weather has been showing 100% chance of thunderstorms all week. Then today, it's suddenly going down. I can get that the models are going to change up as you get better info, but you left yourself no wiggle room when you said that there will be 100% chance earlier in the week.
I've got something on Saturday and the local weather has been showing 100% chance of thunderstorms all week. Then today, it's suddenly going down. I can get that the models are going to change up as you get better info, but you left yourself no wiggle room when you said that there will be 100% chance earlier in the week.
7
Posted on 4/28/16 at 9:32 am to KG6
quote:
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.
Posted on 4/28/16 at 9:34 am to KG6
I 100% agree with you
Posted on 4/28/16 at 9:34 am to KG6
Great job, they are basically never right, yet old people rush home to watch the local news still to hear these clowns spout of their non-sense about the weather.
Posted on 4/28/16 at 9:34 am to KG6
quote:
often times you'll see 0% and then they change it the next day. Why do they do this???? Especially in the extended forecast.
factors change
quote:
100% chance of rain. That means there is absolutely no chance, or that it absolutely will rain without any room for doubt.
I believe a 100% chance of rain doesn't actually mean you specifically will get rain I think it means there is a 100% chance that that somewhere in the viewing area will get rain...but I could be wrong.
Posted on 4/28/16 at 9:38 am to mylsuhat
So to say 100% would mean that the forecaster has 100% confidence that it will rain in 100% of the area? and conversely for 0%
If a guy tells me 90%, then moves it to 50%, I've got no issue with that at all. New data is going to allow you to adjust your confidence levels. But, when you get into the two points I mentioned, you better plan to never move them in my opinion. Because you've stated that without any room for doubt, your forecast will happen.
If a guy tells me 90%, then moves it to 50%, I've got no issue with that at all. New data is going to allow you to adjust your confidence levels. But, when you get into the two points I mentioned, you better plan to never move them in my opinion. Because you've stated that without any room for doubt, your forecast will happen.
Posted on 4/28/16 at 9:40 am to mylsuhat
quote:
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.
Thanks for posting this. Should help with a lot of the misconceptions.
Posted on 4/28/16 at 9:41 am to Topwater Trout
quote:
I believe a 100% chance of rain doesn't actually mean you specifically will get rain I think it means there is a 100% chance that that somewhere in the viewing area will get rain...but I could be wrong.
Correct.
There will be measurable precipitation within a certain geographical area. Which is normally quite large.
Posted on 4/28/16 at 9:43 am to Topwater Trout
quote:
factors change
Again, I totally get that. But if factors still have the possibility to change, then it's impossible to give a 100% chance?
It's April, I realize that it's going to randomly rain. But I've been having races and bike rides that cost 50-150 bucks to participate in and you don't get your money back if it's cancelled due to weather. Sometimes I can wait until the day before to register. Luckily the race this weekend I paid for months ago, so it doesn't matter. But, for the other ones, without missing a beat, it's always been 100% chance of rain early in the week, then on several, by the weekend it's a 0% chance
. Some I go to bed thinking it's going to be pouring when I wake up only to see it getting dropped to 0%. 
Posted on 4/28/16 at 9:53 am to KG6
They are reporters, not meteorologists. They just report the information the computer gives them. That info is fluid because the computers are constantly updating with the changing weather conditions
Posted on 4/28/16 at 9:54 am to KG6
Weatherman: Least accountable "profession" out there
Posted on 4/28/16 at 10:04 am to Hammertime
quote:
That info is fluid because the computers are constantly updating with the changing weather conditions
Again, I have no issue with this fact. What I take issue with is 0% and 100% specifically. Those numbers are absolutes. Even with the definition of CxA that was provided earlier in the thread. You cannot move from an absolute in my opinion, so why state it if "things are constantly changing". Seems like you would say, "I'm extremely confident that it will not rain this weekend, but we still have a lot of time to have things change, so we'll have to leave it at 10% for now".. To me that means, it's probably not going to rain, which is good enough. But when you tell me it's 0%, then I'm thinking it definitely won't rain, and what if I drop a ton of cash on some outdoor activity, only for you to up it to an 80% chance 3 days later!!!???
I just don't see how we accept this.
Posted on 4/28/16 at 10:09 am to KG6
quote:
I've got something on Saturday and the local weather has been showing 100% chance of thunderstorms all week. Then today, it's suddenly going down. I can get that the models are going to change up as you get better info, but you left yourself no wiggle room when you said that there will be 100% chance earlier in the week.
While my computer modeling experience does not involve weather forecasting, I can tell you that all modeling is based around "Garbage in, Garbage out." Essentially, your model's prediction is only as good as the data you use to run it. Looking at a 10-day forecast is taking current weather conditions and atmospheric data, and extrapolating that information with assumptions that the current weather patterns will persist. As you get closer to the target day, current conditions are used to update the prediction. Because the weather is constantly evolving based on a wide range of variables, it is impossible to create 100% accurate predictions.
Scientists use statistical methods to estimate the probability of an event. These types of mathematical principles drive lay people nuts, as they aren't used to dealing so freely with such high levels of uncertainty. Also, as posted above, the rain percentage covers the entire area of the prediction. Essentially it will likely rain somewhere in the predicted area, but that doesn't mean that everyone will get precipitation.
Posted on 4/28/16 at 10:11 am to KG6
quote:
What I take issue with is 0% and 100% specifically. Those numbers are absolutes.
unless there is a hurricane coming, you rarely see 100% on the extended forecast
The guy on the news has 1 or two minute segment they don't have time to break down every variable for you.
Weatherunderground and other sites like it are better place to get info
Posted on 4/28/16 at 10:14 am to Boudreaux35
Posted on 4/28/16 at 10:15 am to KG6
quote:
Again, I have no issue with this fact. What I take issue with is 0% and 100% specifically. Those numbers are absolutes.
Would you rather they pass a standard that only allows them to be able to display 1% and 99% to leave room for doubt? These numbers are being shot out of a computer, I highly doubt anyone is actually taking the time to interpret the models for a basic rain forecast.
Posted on 4/28/16 at 10:18 am to Bmath
quote:
Would you rather they pass a standard that only allows them to be able to display 1% and 99% to leave room for doubt
It's petty as frick, but yes. You can expand on that 1% of statistical doubt. I mean, I personally know better and realize I have to keep on watching the forecast, but it's just something that irks me.
Posted on 4/28/16 at 10:22 am to KG6
quote:Well their models probably do say something like 99.7% chance of rain or something like that. And while I agree it would be nice to get more precision, this will probably only confuse the average person even more unfortunately.
It's petty as frick, but yes. You can expand on that 1% of statistical doubt. I mean, I personally know better and realize I have to keep on watching the forecast, but it's just something that irks me.
Posted on 4/28/16 at 10:39 am to Bmath
Garbage in, garbage out certainly applies but there is also an issue of resolution. They cut the atmosphere into blocks, and the highest resolution models currently can get it down to about 4km and those are typically very short term. So subtle features get missed. The longer forecasting global models don't have that level of resolution.
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