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I heard people say that assuming tomorrow's weather will be the same as today's is as good as, or better, than meteorological models.

  • This book says people assume is the best way

  • This site claims that it's 40% accurate.

Has anyone tested the accuracy of this model and compared it to modern weather predictions?

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  • How accurate it can be would be interesting to see, but both me and likely a lot of other people have seen weather shift rather fast. I've had one point where it rained and was cold the first day, sunny and summer warm the other...
    – Sharain
    Mar 29, 2015 at 13:00
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    @Sharain: Yes, anecdotal data like that is not terribly helpful when the claim is that it is (only) 40% accurate.
    – Oddthinking
    Mar 29, 2015 at 13:27
  • @Oddthinking the claim is also that it is (or was) more accurate than weather models.
    – Sklivvz
    Mar 29, 2015 at 14:19
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    Accuracy in % is not necessarily the most useful metric in weather forecasting. A meteorologist in Saudi Arabia might correctly predict the weather with this model 364 days per year. What makes him (or her? not sure in SA) valuable is predicting the one day that the storm strikes. It also depends on the precision of the forecast. Also, the accuracy of a "tomorrow equals today" model is going to vary tremendously on location.
    – gerrit
    Mar 31, 2015 at 15:22
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    @gerrit: It's also helpful to understand just why this seems to be a good model, when in fact it isn't really useful. In many places, the weather comes in systems that take several days to pass any given location. For instance, here in the western US, a ridge of high pressure can settle in for many days, bringing clear skies & warm temperatures; or Pacific storm systems may take days to pass through. Thus there will be many more 'similar' days within a system than days which transition between them. (See any meteorology text for references.)
    – jamesqf
    Apr 2, 2015 at 21:33

1 Answer 1

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Blogger Randal Olson reproduced a chart from Nate Silver's The Signal and the Noise which in turn has based on data from ForecastWatch.

enter image description here

Ignore the orange line; it is irrelevant for this discussion. (Just for illustration: It is based on a similar idea of predicting that it will be hot on your birthday, because it has been hot on your previous birthdays.)

The blue line represents Persistence - the concept in the question.

The grey line represents commercial quality forecasts.

The higher the line, the worst the estimate.

The blue line is always higher than the grey line - a delta of about 2.5 °F (about 1.5 °C) after 1 day.

Based on this, we can conclude that, although Persistence isn't a terrible model (predicts with an error of only about 5.5 °F, or 3 °C), it performs much more poorly than a professional weather estimate on temperature forecasts.

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  • 1
    Are forecasts by organisations like NOAA "commercial forecasts"?
    – gerrit
    Apr 2, 2015 at 15:03
  • Also, note that this considers only temperature. For most people, precipitation and wind are very much part of weather, and the quality of the "persistence" model might be even worse there.
    – gerrit
    Apr 2, 2015 at 15:07
  • @gerrit Do non-sailors care much about the wind? Assuming the temperature is 'seasonable' I would mostly only want to know whether it's going to rain: and the "persistence" model might be quite good there -- "no rain today and no rain tomorrow" or "rainy today and rainy tomorrow".
    – ChrisW
    Apr 2, 2015 at 15:18
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    @ChrisW In The Netherlands, almost everybody cares about the wind, because if it's windy you might have to leave home 10 minutes earlier for getting to work on time (by bicycle). So it depends on the area.
    – gerrit
    Apr 2, 2015 at 16:18
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    Misleading unless you read the axis? Huh? What are we, illiterates? Would you say a graph comparing prices is also misleading if the cheapest alternative is the line furthest down?
    – gerrit
    Apr 2, 2015 at 17:19

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