Weather Routing with GRIB Files: Reading Models Beyond the Arrows

Every offshore passage now starts with a download. GRIB files, the compressed gridded weather data that flows through satellite connections into apps like PredictWind, Saildocs and LuckGrib, have turned weather routing from a professional service into a cockpit skill. Yet the same files that empower cruisers also mislead them: a clean colour map showing ten knots on the rhumb line has ended more than one passage in a gale that the model smoothed away. The skill is not reading the arrows; it is knowing what the model cannot see, and making decisions with margins the software never suggests.

What a GRIB File Actually Is

A GRIB file is not a forecast drawn by a meteorologist; it is raw output from a numerical model such as the American GFS or the European ECMWF, sampled on a grid, typically every 0.25 to 1 degree, at time steps of three to six hours. Nobody has looked at your specific patch of ocean. The model computes wind and pressure at grid points, and the software interpolates between them. This explains the two classic traps: coastal effects disappear because the grid is coarser than the headland, and small intense systems, tropical squalls and young lows, are smeared into gentler averages. A GRIB showing 18 knots can contain a squall line delivering 35.

Model Wars: GFS versus ECMWF and Why to Carry Both

The European model has earned its reputation: on average its medium-range forecasts verify better, especially for the track and timing of Atlantic lows. The GFS updates four times a day, is free, and often handles tropical convection differently enough to matter. The working habit of careful routers is to download both, flip between them, and treat agreement as confidence and disagreement as a warning. When the two models diverge on day four, the honest conclusion is not that one is wrong; it is that day four is unknowable, and the route should preserve options. Ensemble forecasts, where the model runs dozens of perturbed versions of itself, quantify this directly: a tight cluster of solutions deserves trust, a scattered one demands a wider safety margin.

• Download at least two models on the same cycle and compare before deciding.
• Refresh the files daily; a five-day-old forecast is history, not guidance.
• Watch the model run-to-run trend: a low that deepens on three consecutive cycles is real.
• Treat any wind barb over 30 knots as a zone to avoid entirely, not to thread.

The Routing Run and Its Honest Limits

Routing software layers the boat's polar performance over the GRIB and computes a fastest track. The output is seductive and needs three corrections. First, polars assume flat water; a boat pounding into two-metre seas loses 20 to 30 percent of its speed, so upwind routes are systematically over-promised. Second, the optimum for speed is rarely the optimum for a cruising crew; most skippers add constraints, no apparent wind above 25 knots, no sustained upwind legs, and accept a day longer for a week kinder. Third, the route is only as good as the day-seven forecast, which means it is a hypothesis to be re-examined at every download, not a plan to be defended.

The Instrument That Never Lies

Every routing article should end with the barometer. The models predict; the glass reports. A fall of two millibars in three hours means the weather is arriving faster than the download said, whatever the download said. Crews that log pressure every watch, note the cloud sequence, and compare reality against the forecast build the calibration that no app provides: knowing when their stretch of ocean follows the model and when it writes its own script. GRIBs have made ocean routing safer, but only for the sailor who remembers that the file is a guess, and the sky overhead is the fact.