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Old 23-04-2009, 17:11   #1
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Grib Files Time Question

Can someone confirm that the grib US files, when displayed on Maxsea, show up in universal time and not local time?

Thank you
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Old 30-01-2010, 06:58   #2
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Resurrecting old question. . . . I have the same question about viewing GRIBs in Viewfax.

I've subscribed to Saildocs sending me a GRIB for the Bahamas each morning at 5AM local time, or 1000z, asking for WIND at 3, 6, 12, 24, 36, 48, 72, and 96 hours out.

What I receive from Saildocs says "Grib extracted from file gfs100130-00z.grb dated 2010/01/30 04:38:34" So that sounds like local time.

When I view the GRIB in viewfax, the top line says "GRIB valid 2010/01/30 03:00" and the little time stamp that accompanies the pulldown menu of forward times also says that, e.g., as I step through 3, 6, 12 etc hours ahead, it will say "2010/01/30 . . . 06:00 . . . 12:00 etc."

So are those UTC times in the GRIB viewer? I normally would assume so, but it seems strange that a GRIB arriving at 5AM local time would start retroactively at 0300z, or 10PM the night before. It would certainly be convenient for viewing and planning if those times were local. But I'd guess that since many GRIBs span multiple time zones, the software is set for UTC time only. (?)

So then my question becomes: How do I construct a GRIB subscription request that will actually start each day at 5AM or so?

Any GRIB gurus, please advise!
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Old 03-02-2010, 11:27   #3
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I posed my question to Jim Corenman, who wrote the Viewfax software. Here's his very helpful reply:

Everything is UTC.
>I've subscribed to Saildocs sending me a GRIB for the Bahamas each morning
>at 5AM local time, or 1000z
>...
>What I receive from Saildocs says, for instance, "Grib extracted from file
>gfs100203-00z.grb dated 2010/02/03 04:37:48"
That is correct. Please review the grib info from Saildocs (www.saildocs.com/gribinfo or send a blank email to: gribinfo@saildocs.com). The GFS model is updated by NOAA every 6 hours, generally available about 5 hours after the synoptic (zero-hour forecast) time, i.e. available by 0500, 1100, 1700 and 2300 utc.

So your request for a grib at 1000z is coming from the 00z model-run which became available at 0500z (04:37 in the example you sent).

If you wait an hour then you will get the next model-run. Conversely you can get the same file as early as 0500z.

When you send your request use "GFS:..." and not "Grib:...", the latter fudges the forecast-times to be relative to 00z which adds another layer of confusion.

Good sailing,
Jim
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