Mark also wonders about weather forecasts on his blog. He says:
If we have a prediction that is inaccurate, displays sudden swings, and makes forecasts wildly divergent from its competitors, that raises some questions.
As many of us have discovered, the websites that carry weather forecasts seem to be wrong most of the time. But "wrong" is hard to define. In fact, even divining the meaning of the probability of rainfall is hard, as I discussed before.
My other friend, with whom I mused about the accuracy of Waze's estimated driving time (link), is on the verge of giving up on Waze completely. Last week, he told me that Waze is not just messing up the time estimation, but it's also getting the routing wrong more and more frequently.
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For weather, it seems that forecasts can be quite accurate when they are expressed using ranges of time. An example of such a forecast is a noreaster might hit the Northeast within the next five days, bringing temperatures down to around freezing point. In my experience, that statement has a high chance of being correct. But... if the forecast was stated as the temperature will drop to freezing point in NYC on Wednesday afternoon between 2 and 3 pm because of a noreaster, my experience is it's almost always wrong. The temperature plunge would indeed come around but the timing would be off.
The above forecaster thinks temperature will drop below freezing this coming Sunday. The timing of that forecast is probably wrong. If we look at that, and modify the forecast to Sunday plus/minus 2 days, i.e. some time between Friday and Tuesday, then we'd be more confident in its accuracy. Less precision is better!
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Then comes Mark's issue. What if the forecaster continuously updates the forecast, so by Friday morning, the temperature drop is pushed back to Monday 4 pm, then by Monday 2 pm, it's further delayed to 6 pm, and by 5 pm, it's now slated for Tuesday 9 am, etc. etc. This is indeed what these weather websites do. Eventually, their forecast is "correct".
Because weather is a "continuous" phenomenon mostly, that is to say, the noreaster isn't going to emerge instantaneously on top of NYC, such a continuously-updated prediction methodology is guaranteed to become accurate eventually.
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