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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 9:53 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 10:09 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.
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