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Old 16-01-2010, 08:29   #1
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Grib Files and Grib Fantasies

When we make a passage on Exit Only, we download grib files every day. I tend to get large files (much larger than our sailing area) to get a feeling for the general wind patterns in our section of the world. I feel that the overall wind patterns for a region make me more situationally aware about what's going on around me.

I will download files out for a one week period of time just to see what the computer models think about wind speed and direction in the future. Most of our sailing has been in the tradewinds outside of hurricane season, and so the wind patterns are often well established. Nevertheless, we find that the grib files are generally good in predicting wind for only two to three days, and out further than that, we don't rely on them. It may be that the grib files are better at long term predictions at some lattitudes and locations than others.

In my experience, the Grib files become grib fantasies after three days, and sometimes less than three days.

In your experience and in your location, when do you feel that Grib files become grib fantasies and no longer accurately predict wind speed and direction?
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Old 16-01-2010, 09:28   #2
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I use grib files in conjunction with weatherfax, other forecasting models (e.g. SailFlow), marine weather observations and forecasts on the VHF, and my own observations of the sky, barometer,etc..

The grib files not a forecast, not a prediction. They are conditional and conditions change. Any forecast gets tenuous as you get further out.
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Old 16-01-2010, 11:20   #3
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Quote:
Originally Posted by maxingout View Post
In my experience, the Grib files become grib fantasies after three days, and sometimes less than three days.

In your experience and in your location, when do you feel that Grib files become grib fantasies and no longer accurately predict wind speed and direction?
That's a know "feature" of weather forecasting / climate models and not specific to gribs, first few days are highly accurate now, after day 3 it becomes guesswork
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Old 16-01-2010, 15:51   #4
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Quote:
Originally Posted by maxingout View Post

In my experience, the Grib files become grib fantasies after three days, and sometimes less than three days.
We use a similar technique to yours but for the general picture I use we-fax. We never rely solely on gribs, unless radio-fax is unavailable.

What I like about gribs is that they better visualise what happens during passage of fronts - how the wind will behave - general trends - like whether it will veer or back and how much stronger/lighter it will become (not realy telling how strong it will blow but showing the trend).

If you look at model descriptions by each grib source you will notice they promise nothing more than fantasies beyond 3 days. Sure they are educated fantasies, or guesses. The same 'fantasy' effect, however, affects radiofax, text forecast and all the rest of we-stuff.

My observations re gribs are as follows:
- very accurate offshore within 24 hrs (trends, not values),
- reliable inshore, but local phenomena have to be allowed for.

In any case, since we only have gribs in port, we have them a couple of times a day and so we will use the most recent one to plan departure. We will never use a 3-day-old grib to plan anything, so their accuracy at the 3 day horizon is of no importance (but I am sure you were not talking about this aspect, so this is just a by-comment).

Long live gribs!
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Old 16-01-2010, 15:54   #5
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That's a know "feature" of weather forecasting / climate models and not specific to gribs, first few days are highly accurate now, after day 3 it becomes guesswork
INDEED, though the accuracy of 3 - 5 day forecasting has been steadily improving.
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