Reading GRIB files: the guide most sailors skip
GRIB data is the closest thing to a forecast you can hold in your hands offshore. Here's how to read it without getting lost in the numbers.
GRIB (GRIdded Binary) files are the raw output of numerical weather prediction models. Where a forecast website shows you a curated, simplified view, a GRIB file shows you the model output directly — wind speed and direction, pressure, wave height, precipitation, all interpolated onto a grid. For offshore passages, they are the closest thing to a forecast you can actually trust beyond 48 hours.
What a GRIB file actually contains
A GRIB file is a sequence of binary records, each containing a single meteorological parameter at a single time step. The parameters most relevant to sailors are:
- UGRD / VGRD — the U and V components of wind (east-west and north-south). Your viewer converts these to speed and direction.
- GUST — surface wind gust. Often meaningfully higher than the sustained wind.
- PRMSL — mean sea level pressure. The pressure gradient is what drives the wind.
- HTSGW — significant wave height. This is the average of the highest third of waves — real waves will be bigger.
- PERPW / DIRPW — dominant wave period and direction. Period matters as much as height.
Which model to use
The two models sailors use most are GFS (NOAA, American) and ECMWF (European). There is a third worth knowing: ICON (German, DWD), which performs well in European waters.
GFS updates four times daily and is freely available. ECMWF is widely regarded as the more accurate model globally but the full resolution output requires a subscription (via services like PredictWind or Windy Premium). For most coastal and offshore use, GFS is excellent. For a transatlantic passage where you want the best possible medium-range forecast, ECMWF is worth the cost.
How to read the output
The trap most sailors fall into is treating the model output as a point forecast. It is not. A GRIB file is a grid — each cell represents a large area, often 25–50 km across. Real wind is not uniform across 25 km. Coastlines, islands, and local thermal effects will all produce winds that differ from the model.
The rule: use GRIB for the trend, use observations for the moment.
When you open a GRIB file, you are looking for:
- The pressure pattern — where are the highs and lows? Where is the gradient strongest?
- The wind direction — is it consistent, or is there a significant shift in the next 24 hours?
- The wave height and period — a 3m sea in a 12-second swell is very different to a 3m sea in a 6-second chop.
Tools
Expedition is the benchmark offshore routing software and handles GRIBs natively. OpenCPN with the weather routing plugin is free and capable. For mobile, PredictWind Offshore and Windy both have excellent GRIB viewers.
Sidon’s map integrates Windy’s GRIB visualisation directly — you can overlay the forecast on your route and see when you will be in the strongest wind without switching apps.
The model output is not perfect. Nothing is. But it is the best tool we have for understanding what the atmosphere is about to do — and for offshore passages, understanding that is the job.
