About this page
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
- Free sources of GRIB files
- Viewers for GRIB Files
- A review of Services
- Getting GRIBs from Saildocs over HF
- The latest Saildocs Newsletters
- Setting up HF radio for GRIB and other purposes
- A Warning
On this page
- Where they come from
- Large scale forecasts
- How to get the data
- Meso-scale forecasts
- A cautionary note
- File Sizes
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
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 US Navy NOGAPS (Navy Operational Global Atmospheric Prediction System) as a back-up.
The NOAA NCEP (National Center for Environmntal Prediction) is, along with the UK and ECMWF, 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. The UK Global model calculates on a 3-D grid with horizontal spacing of about 25 km _(1/4 degree or about 15 nautical miles). The smallest feature that they can represent – geographical and meteorological – is about five times the grid spacing ie 60 to 70 miles.
NOTE Although the GFS uses a slightly larger grid of about 27 km, the data are only made available by NOAA on a 1/2 degree grid ie around 30 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 FTP (File Transfer Protocol)
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 ot 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
These calculate on smaller scales than is usually represented on synoptic charts. The UK Met Office uses a 12 km grid for an area stretching from the eastern US across to Europe to help the production of more detailed forecasts for up to about 36 hours ahead. A 4 km grid covering the UK only is used for forecasts for the next few hours. Experimentally, they are now using a 1.5 km grid to replace the 4 km model after validation.
0nly National Weather Services have the capacity to generate 4-D data analyses combining a short term detailed prediction with all, or much, of the vast quantities of data available. 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|Forecast Accuracy Limitations. ting GRIB services.
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 .
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 firstname.lastname@example.org, this will return the how-to document (about 5K).