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What GRIB files are and a word of warning.


GRIB files can be obtained free and many sailors now use them as a low cost way of getting a great deal of weather forecast information. There is much misunderstanding of GRIBs, some of it fostered by those selling GRIB services and products. I hope that this and the associated pages will help users to know what they are getting, how to assess what is available at no cost and which services are worth paying for.

On other pages of this site see

On this page

Where GRIBs originate and what are they?

National Met Services (NMS) use GRIB code (Gridded Information in Binary) to store and exchange forecast data. However, most do not release them for general use. Exceptions are

  • US National Weather Service (NWS) – a part of NOAA,
  • US Navy
  • Canadian Met Service
  • Météo France

These all make data available to anyone with the computer resources to handle massive data sets. Most free sources of GRIB data for anywhere in the world use the US GFS (General Forecast System) with the others as a back-ups.

Along with ECMWF and the UK Met Office, the NOAA NCEP (National Center for Environmental Prediction) is, a leading player in NWP (Numerical Weather Prediction.)

To see what GRIB files do in practice, see the GRIB examples page. This show forecasts for days 1 to 7 compared to actual wind patterns.

Large scale forecasts

Forecasting for more than a day or so ahead requires global models partly because weather can travel a long way in 24 hours and, more importantly, because there are long distance connections; weather occurring in one area can affect what happens far away.

National Met Services usually use grid lengths (spacing) of about 1/4 degree. The smallest feature that they can represent – geographical and meteorological – is about five times the grid spacing ie over 120 km or 70 miles.

NOTE Although the GFS uses a slightly larger grid of about 1.8 degree lat/lon, 13 km, the data are only made available by NOAA on a 1/4 degree grid ie around `15 mile spacing.

This has implications for those who use the degraded data as a basis for meso-scale models without using the mass of data available- see below.

Getting the information

There are many routes by which we can get global forecasts (GFS) and most are free to the user, except for communications costs. Examples that provide wind vectors, and other information, from GRIB files in chart form are –

By email.

  • Saildocs, Not the easiest to use. The most flexible and cheapest in comunications for repeat usage.
  • MailASail

By FTP (File Transfer Protocol)

  • ZyGrib A more comprehensive free service with more data options. For Windows users. For Window, Mac OS-X, Linux..
  • Tablet Apps. There are several offering a range of options for both Apple and Android devices.

By browser.

Easy to use, provided you have the bandwidth.

Weatheronline, PassageWeather, Windfinder, YBW. and some others are included in a listing of GRIB services on another page.

Some services that seem to offer more are

WindGuru. (free. Interpolates the GFS to specific locations).

MovingWeather (on repayment. Zooms in from the GFS to whatever area you ask for.

XCWeather (interpolates forecasts to observing site locations).

These simply interpolate between the values on the 1/2 degree grid. There are no, and can be no, differences in the quality of the product. .In fact, they give a false impression of precision.

GRIB files received by email or FTP can usually be viewed using a variety of viewers. each has its pros and cons. See my GRIB viewer page for some alternatives.

Small scale forecasts – meso-scale models

Meso-scale models set out to predict on scales smaller than can be seen on synoptic charts. Grid lengths can be down to around 1 km.

Data used

National Weather Services have the capacity to generate 4-D data analyses combining a short term detailed prediction with high resolution data. For an idea on the types and amount of data used, browse the ECMWF Data Coverage pages.

The UK and France use these forecasts in "ensembles". That is, they run a forecast several times with small variations in the analysis to counter uncertainties in the data. Rather than use meso-scale forecasts in a deterministic manner, it is more sensible and prudent to use them to determine probabilities. I have not made a systematic study of meso-scale output. However, I have never found that they provided any more useful information than the US GFS combined with whatever GMDSS forecast available. I can only advise those looking at commercial GRIB services to ask themselves if they really are getting better guidance and not just more data..

For more on Numerical weather Prediction see my pages on NWP, the importance of Grid Length and [[Forecast-Accuracy- Limitations. ting GRIB services.


File Sizes

A file of GRIB data at a 1 degree latitude/longitude spacing covering the area 30N to 45N, 10W to 15E, giving wind arrows for eight times ie 12, 24, 36, 48, 60, 72, 96, 120 hours ahead (12 hourly to 3 days and 24 hourly to 5 days) would be 15x25x3x8 ie 9000 values.

The file containing these data is less than 15 kb. Output for half the area would be half as much ie less than 8 kb. If only winds had been requested, then file sizes would be 2/3 of these values. An example of one of the eight charts received in this way is shown here using the Viewfax software.

The isobars shown here are at a rather fierce 2 hPa (mb) spacing. There are options to change colours, scale and isobar spacing.

More information on Saildocs is available by sending an email to info@saildocs.com, this will return the how-to document (about 5K).

Cautionary NOTE -

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 .

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