Is grib weather data reliable?

One of the newest pieces of information used by mariners to get a fix on weather is the gridded binary (grib) file. Grib data is a weather data format that, in a small file, gives you a lot of information. These files are displayed, by some navigation software or by a grib file reader, as a surface-pressure chart with wind arrows.

Navigation programs are using this data to overlay weather features onto charts. Some websites also use the data as weather forecasts. You need to know the quality of grib file forecasts is not always what it seems.

The reason grib data has became so popular is that the file size for the amount of data is very small. This makes it very easy to download via email at sea. The radio email systems are slow, and the satellite systems are expensive per minute, so in this case, size is important. You can get a three-day grib file forecast smaller than a single weatherfax chart.

I became a big fan of this data and began using it as a primary source. I was delivering a boat from San Diego to San Francisco. I had driven straight from a sailing trip in Ensenada, Mexico, and had not received any weather information. When asked by the crew for the plan, I stated we should round Point Conception and Point Sur during early morning hours. I figured this would give us the best chance of lighter winds at these areas known for turning back boats. On the way to the fuel dock I downloaded a grib file. After studying it, I surfaced from the cabin to announce a change of plan. We needed to round the points by midday. The grib data showed a high-pressure area over the coast, giving us flat seas and light air for most of the trip. With the performance boat we were on and its minimal fuel capacity, we would need to sail a lot of this trip. As the grib data forecasted, we were able to sail in a 10- to 15-knot NW breeze around these points well known as boat breakers. Knowing this saved us hours of motoring. Collecting good data not only keeps you out of extreme conditions but can give you a pleasant sail when another route could mean getting beat up or motoring in a calm.

Since then I have found grib files don’t always have the most accurate information; many times they miss the mark on higher wind speeds. You really need to practice with these files and learn their weaknesses. For example, from a polar-orbiting-satellite image, I noticed a storm developing off Cabo San Lucas, Mexico. I decided to pull down some grib files, as well as other files, to see the picture from the National Weather Service point of view. The grib files did not reveal the higher winds.

Moral of the story? Don’t use any single weather source for decision making. As you can see, you could have made a big mistake depending on just grib data in this instance. Make a personal note of the grib file’s lack of ability to show frontal systems. Practice collecting and comparing weather data from several sources.

Grib files aren’t useless, of course. Grib data provides a lot of information for very little time on email. Mix this data with your text and charts to plan a trip. It is also the only information you might have when you get to places like the west coast of South America, where, for some reason, people have a hard time getting regular forecasts, and the information doesn’t show up on the Internet with great regularity. Some friends of mine, while on their way to the South Pacific, decided to head down to see the Galapagos and Easter Island. An old salt, the captain had a dedicated weatherfax and was very proficient at using it. As he approached the equator he started tuning the weatherfax to the Chilean broadcast. I soon had an email asking for help since the Chileans hadn’t been broadcasting. So some data is better than none. Learn to use them but remember their limitations.

A Coast Guard licensed master, Ed Witts lives in Northern California aboard a 48-foot aluminum performance cruiser. He can be reached at

Chris Parker responds:

As Ed Witts attests, many people get into trouble relying solely on wind and pressure grib data from the global GFS/AVN model. But why? This is probably the best worldwide computer forecast model, and it is the one used by most grib data providers. This model does an amazingly good job of forecasting dozens of atmospheric parameters, in addition to wind and pressure, on a large, synoptic scale.

The type of wind forecast generated is often referred to as a gradient-wind forecast. It basically predicts wind speed and direction as driven by changes in atmospheric pressure over distance. You should rely on GFS/AVN grib data as your sole wind forecast only when there are no local or regional surface or upper-air weather events taking place. On the vast majority of days, due to the precision of grib forecasts, GFS/AVN wind speed and direction predictions are better than you will get from most forecasting tools.

The problem arises when locally severe weather strikes. The GFS/AVN model is written to actually ignore locally driven weather events, in order to provide a clearer large-scale picture. Grib data is available to predict these local and regional weather events in many parts of the world, and most of NOAA’s coastal U.S. forecasts are available in grib format.

So the issue is not that grib data is inherently unreliable. That would be like saying television is a poor means of transmitting news and entertainment because the show on the channel you’re watching doesn’t suit you.

Grib data, when used with a capable grib viewer software program, can provide an interactive graphical and text forecast for multiple time periods, and with pinpoint precision, in about a tenth the file size of familiar weatherfax charts. The solution is to find and use the appropriate data with a good software viewer. If you have any doubts, confirm the grib predictions with an official forecast.

By Ocean Navigator