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What is a GRIB Weather File?

What is a GRIB Weather File?

GRIB Weather FileGridded Binary data files (GRIB) are output files generated by computer forecasting models. They are tremendously more compact than regular weather charts and because of their size are very well suited for download via wireless means. The largest producer of GRIB weather is NOAA although other organizations such as the military and research institutions also generate GRIB files. NOAA GRIB files are the best ones suited for marine forecasts.

Be aware that these forecasts are published without human intervention. Human forecasters do not review the data before it’s published and therefore some knowledge and awareness is required by the end user. Also note that NOAA uses several models to generate GRIB files and it is the users responsibility to know the strengths and weaknesses of each forecast model before using them to plan their travels.

It is best to familiarize oneself with the GRIB files from each model for the area you travel in and compare these forecasts with other reliable weather sources before depending on them to make predictions. You will find that GRIB forecasts can be extremely accurate and in remote parts of the world possibly the only source of weather forecasting. We have had reports from users in the Beagle channel in South America that claim that the GRIB forecasts received from GMN are “spookily accurate”. None the less a keep in mind that a forecast is a “forecast” and forecasts have been known to inaccurately predict the future.

GFS/AVN vs. Wwave3 GRIB Files

NOAA produces two GRIB formats suitable for Ocean wind forecasts. These GRIB formats are produced by the Global Forecast System known as GFS/AVN and the wave watch model known as wwatch3. Both models generate similar forecasts but there are a few differences enumerated below:

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  • Generates wind forecasts at Sea Level taking into account wave and current effects.
  • Forecasts are only for Ocean waters. The model does not generate forecasts for interior bodies of water such as the Mediterranean, Baltic, Black Sea, and Great Lakes.
  • Does not forecast winds over land. This results in smaller GRIB files which is a tremendous advantage to wireless users.
  • Generates forecasts for significant wave height, mean wave direction, mean wave period, peak wave period, peak wave direction, wind sea direction, and wind sea period.
  • Forecasts are produced in 3 hour intervals for 7 days (168 Hours).
  • Model is run 4 times a day.
  • Data generated on 1×1.25 degree grids.
  • Produces wind forecasts for the globe (Ocean and Land) at 10m. Wave action is not taken into effect.
  • Forecasts are produced in 12 hour intervals to 7 days and then in 24 hour intervals to 14 days.
  • Generates forecasts for pressure at mean sea level, 500 mb pressure height, surface temperature at 10m, relative humidity, precipitation, plus much more.
  • Model is run 4 times a day.
  • Data generated on 1×1 or 2.5×2.5 grids.

GMN uses wwave3 GRIB data to generate Ocean wind and wave forecasts and GFS/AVN for pressure, temperature, 500mb. As a public service GMN delivers wwave3 wind forecasts at no charge via ftp, http, and email. No subscription is necessary to access the wind GRIB files. Users wishing additional data should consider using our premier data download product WeatherNet.

Advantages of GMN GRIB Over Other Providers

  • GMN GRIB files are considerably smaller. Almost by a factor of 2.
  • More forecasts are packed into the files. You get forecasts for every 6 hours vs. 12 hour forecasts for most others.
  • E-Mail robot syntax is simpler supporting custom regions and automatic scheduling.
  • Wwave3 GRIB files used for wind forecasts.
  • The only disadvantage wwave3 has over GFS/AVN for wind forecasts is that it does not create wind data for interior bodies of water such as the Med, Baltic, Black Sea, etc… For these you will have to use the GFS/AVN GRIB files.

Note: The GMN mail robot will send you wwave3 GRIBs if you request a custom region or one of the static regions which does not contain an “interior” body of water. If your request a static region which contains an “interior” body of water such as the Med or Great Lakes you will receive an AVN GRIB file.

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