What is GRIB Code?

GRIB (Gridded Information in Binary) is a generic name for code forms devised by the World Meteorological Organisation to facilitate the transfer of large data sets between national meteorological services. Most of the data are output from Numerical Weather Prediction (NWP) models.

Data analyses and prognoses are stored as numbers on a latitude/longitude grid using binary format to minimise file size. Software can transfer subsets of the data to charts with topography and lines of latitude/longitude. When sailors refer to GRIBs or GRIB files, they usually mean wind or winds and isobars but GRIB files can be of most meteorological parameter, including swell.

Forecasts of whatever elements are required, for several days ahead, can be contained in small email attachment of a few kb compared to one synoptic chart from a website taking 50-80 kb. Processing into chart form is usually effected using standard software off line. This page briefly describes GRIB output from the US National Weather Service and how to view them.

There are free services from SailDocs, GMN, MailASail, WWW.GRIB.US and web browsers. There are charged for services such as the Météo France Grib-And-Objective-Forecasts-Reviewed, Movingweather, and ProGRIB. There are also services linked to chart plotters.

The major, basic source is the US NWS Numerical Weather Prediction model, the GFS (Global Forecasting System) which calculates how the weather is expected to develop for several days ahead (up to around 15)., For this purpose it defines the atmosphere on a three dimensional grid of points. The lowest level of this model is at about 20 metres above ground (or sea) level. This is at a height where the pressure is about 3 hPa (mb) lower than at the surface of the earth.

From the forecast wind at this level and using algorithms embodying surface roughness and stability of the air, the NWS derives a forecast at 10 metres. The forecast winds are also used to drive a wave model known as the MAR Operational Wave Model (aka Wwave3).

Alternative basic sources are the US Navy NOGAPS and the Canadian Met Service.

NWS wind data in GRIB format are available, free, from several sources. Most, such as Saildocs, provide the wind at the 10 metre level directly from the GFS. One service, GMN provides wind data from the output of Wwave3. These are derived using a slightly different algorithm. The situation is somewhat confused because the GFS output is available on a #

1 deg x 1 deg grid while the wwave3 output is on a 1 deg x 1.5 deg grid; this involves some spatial interpolation. The differences between the 10 m (GFS) winds and the winds output from MMAR will not be great and probably well within the noise level of the actual wind on many occasions. Remember that the atmosphere does not know itself to better than one Beaufort force. US NWS output can be obtained free of charge from some sources and on repayment from others. Other, more extensive services on repayment are available from a variety of sources see my page Which GRIB Service?.

A summary of free data sources is on another page.

From whatever source, it is important to remember that GRIB files are computer generated forecast files from a National Weather Service computer. These are sent without review, and are offered on an as-is basis. There is no assurance that the data are available, accurate or correct. Systems providing information and the computer models are automated and subject to a variety of failures and errors. By using the data, users acknowledge and agree to these limitations.

It should be remembered that human forecasters can still add value to computer model output. Consequently, the prudent Sailor will only use GRIB products in the short term (say up to 24 or 36 hours) in the

light of other sources of information eg text forecasts, forecast charts generated by man-machine-mix, from the UK Met Office for example, or satellite pictures. For longer term predictions - say over 48 hours to five or six days, the basic computer output is unlikely to be improved upon by a human to any great. However, when preparing forecast charts at 4 and 5 days ahead. forecasters at the UK Met Office do look at other forecasts from other centres and try to produce a "best" forecast from a consensus point of view. # For a very sensible appraisal of the use of ANY GRIB based forecast, send a blank email to GribWarning@saildocs.com .