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Darth Gerbil
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Predicting the weather

Tue Feb 11, 2014 11:57 pm

I just checked two weather sites to see tomorrow's weather for my area. One site says it will be 30*, the other says 31*. Both of those get the point across, that tomorrow's high will straddle the freezing mark. It's curious, though, that there's any difference at all between the predictions. I assume there's only a handful of satellites gathering weather data, and a certain number of weather stations around the country equipped with radar and other high tech weather tools, so all weather stations pull data from the same few sources, right? Maybe I'm wrong, and each weather outlet has its own multi-billion dollar network of radar stations and satellites from which to determine the weather?

Most of the time different outlets are in the same ballpark, but how are there any differences at all?
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Re: Predicting the weather

Wed Feb 12, 2014 2:29 am

I don't know about the radars, but a basic weather station can consist of just a thermometer, a barometer, and a wind meter and still give very useful data. Temperature in particular is cheap and easy to measure, so local news channels tend to have their own sources in addition to getting data from NOAA and others.

In addition to that, they may not all be predicting the temperature for exactly the same location, and they probably tweak their weather models differently (possibly using entirely different software), so it's no surprise that they don't report exactly the same forecast.

For best results, look at as many different predictions as you can and average them.
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Re: Predicting the weather

Wed Feb 12, 2014 4:04 am

Even if the different weather sites are all using the NWS as their source, there are plausible reasons for minor differences:

- NWS forecasts can cover a fairly small geographic area. For example, the NWS site is currently forecasting an overnight low that differs by one degree between where I live (on the edge of town), and the center of town about 5 miles away. Maybe slight differences in location databases are causing different sites to pull different NWS forecasts if you are near the boundary between two zones, or maybe some of the sites are averaging forecasts for multiple zones in an area.

- The forecast can change, and different sites may be snapshotting the data at different times.

- NWS data isn't perfect; it is possible that some sites do their own interpretation/correction. For example, the NWS forecast for the center of town here is predicting an overnight low that is 5 degrees above the current reported temperature. Clearly the prediction was wrong, and the error is of a nature that could've been easily caught with an automated script. I've seen other obvious contradictions in forecasts, e.g. a prediction still indicating a 50% chance of rain when it has been raining for a couple of hours.
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Re: Predicting the weather

Wed Feb 12, 2014 6:33 am

just brew it! wrote:
The forecast can change, and different sites may be snapshotting the data at different times.

Yeah, was going to suggest this. Some sites likely update their site data more or less frequently than others.
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Re: Predicting the weather

Wed Feb 12, 2014 2:34 pm

There are many different models and sources of data. I've been following lately for my weather source of choice and on some of his videos he shows the models and talks about when the data comes in, etc. One of the people at work pays for premium access and the video I watched part of was 14 minutes and was even more informitive. He went into each model and what his opinon of it was. Very informitive for us Hoosiers.
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Re: Predicting the weather

Wed Feb 12, 2014 2:45 pm

Also take into account that they may get a possible range... the forecaster him/her-self than needs to pick one number to satisfy the masses.
they may see 29-32 as the possible range.
one may pick high, another average.

Also some pride themselves on knowing the local area a bit better... eg. knowing that the river that runs through the center of town makes the town a degree or so warmer in winter.
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Re: Predicting the weather

Wed Feb 12, 2014 3:13 pm

Predicting tomorrow night's low is a lot more complicated than you might think. Even if you guess the high temperature perfectly, there are dozens of parameters that influence night time cooling. Humidity, sky cover, surface winds, etc. And each of these is, in turn, influenced by larger scale effects. You'll need to accurately forecast when those high clouds will evaporate, how much surface wind the approaching low will generate, and how strongly that will mix the surface air with (usually drier and warmer) air aloft. Etc. Etc. Etc....

While people are still quite good at teasing subtle details out of the data (see for an idea of the parameters to be considered), most news/teevee forecasting is done via computer models. Between the various weather service branches, dozens of colleges, and international sources, there are literally hundreds of models in various stages of tune. All are wildly complex ,receiving data from all manner of non-standardized sources. Even if two models are very closely related, if one is getting a specific data set from a different source than another, their output will naturally vary. News/TV don't use anywhere near all of these models, but even the 'top 5' computer predictions will vary from one another.

Predicting tomorrow night's low is relatively easy, and forecasts are generally good to within a few degrees. But now try to predict the same temperature a week in advance. Chaos! As the fore-cast time increases, the models start to diverge rapidly. Minor differences in data or computing results rapidly compound in an exponential manner. By two weeks, the programs are pretty much 'just guessing.'
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Re: Predicting the weather

Wed Feb 12, 2014 3:19 pm

Something like this would give you an idea of what's involved. ... /index.htm

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