A Warning about Using GRIB Files

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Warnings on the use of GRIB files and computer models. Perhaps a general warning.

A Note on GRIB Files and models

Output from GRIB files can look very impressive but it is necessary to remember the warning on my main GRIB page. Many GRIB file services simply relay output from NWP models, usually, but not always, the US General Forecast System (GFS) model. Therefore, none will be better than others in terms of reliability nor will any give more "accurate" forecasts than another.

There will be differences in presentation, ease of access and in the data offered, whether it be from a commercial service or a free one. All operational weather forecast services are subject to occasional failures. Therefore, from time to time the forecast may not be the most up to date. Check with your supplier what they do in such eventualities. Also, check how often they update their output. The GFS is run four times a day but some companies only provide output twice a day.

NWP models are not and never will be precise. Users are recommended to read my NWP page to get some idea about the limitations of weather prediction. All these factors should be borne in mind when using any forecast and, particularly, one where there is no vetting or modification by a human forecaster. That is the case for all GRIB services.

As computer power increases, so meteorologists can become more sophisticated in their numerical modelling. To read about the differences between Global models such as the US GFS and the various meso-scale models, please go to the NWP page.

Finally GRIB products are only one source of forecasts that the thinking sailor should use. Never go to sea without receiving and studying forecasts issued under the GMDSS. Crude and broad brush they may be, but they do have the benefit of experienced human interpretation. Amendments to GMDSS forecasts are broadcast on voice radio, NAVTEX or INMARSAT-C. There is no equivalent broadcast of GRIB amendment when at sea.

GRIB forecasts or forecasts derived from them should never be used in isolation. They can help to amplify a GMDSS forecast, in the same way as synoptic charts. I suggest that they are most valuable as planning tools over the next five or six days. That is how I use them.

In particular, it is important to remember that any model can, at best represent weather on a scale of about 5 x the grid length. For the GFS, this is about 100 to 150 NM. As a result, winds speeds over the open sea will be under-forecast by about one Beaufort force or 20%.

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