How to read marine forecasts and GRIB files for passage planning
No single product gives you the entire picture. The text forecast, the GRIB file, and the surface analysis chart each show one part of the weather. Each one leaves out something the others cover. Read only one and you sail with a gap you cannot see.
This page covers the three forecast products that drive a passage plan: what each one is, what it shows and what it leaves out, and how to read them together so the gaps in one are covered by the others.
Read them together, not one at a time.
Each product has a job, and the jobs do not overlap. The surface analysis chart shows the systems. It has the highs, the lows, and the fronts, so you can see what is driving your weather and watch it coming. The GRIB file gives you the hour-by-hour detail. It is raw model output on a fine grid, so you can scroll forward and read the wind and waves at each point along your route. The text forecast gives you the official limits and the warnings. A forecaster writes it. It is conservative, and it is where a gale or storm warning lives.
So the chart is for the systems, the GRIB is for the detail, and the text forecast is for the official limits and warnings. Cross-check the three against each other. When they agree, you get a confident forecast and can plan against it. When they disagree, you have found the uncertainty in the forecast. That is worth knowing before you leave.
The six things you read before a passage.
These are the products that go into a passage plan. Learn what each one is for and the forecast stops being a single number and becomes a picture you can trust.
- Text or zone marine forecasts: Issued by forecasters in plain language for a named zone. They are deliberately conservative and give the official wind and sea limits and the warnings. The trade-off is that they are coarse in space and time, covering a wide area in broad terms.
- GRIB files: Raw model output on a grid, fine in resolution and updated hour by hour. The strength is detail along your exact route. The limit is that there is no human judgment in it and no fronts drawn, and it is only ever as good as the model behind it.
- Surface analysis and synoptic charts: The map of highs, lows, and fronts. This shows why the wind is doing what it is doing, the systems that decide whether conditions build or ease. It is the best single tool for seeing a system coming before it reaches you.
- Coastal vs offshore and high-seas products: Know which product covers your waters. A coastal forecast stops at the coastal zone. Once you leave it, the offshore and high-seas text products carry the official picture across open water, and they matter more the farther out you go.
- Warnings and bulletins: Gale, storm, and special marine warnings sit on top of the routine forecast. A forecaster issues them when conditions cross a threshold. Treat them as a hard line and never plan a passage that runs past one.
- Putting them together: Chart for the systems, GRIB for the detail, text for the official limits and warnings. None of them is enough alone. The skill is reading all three, cross-checking them, and noticing where they disagree.
What each product is good for, and where it falls short.
Every product has a strength and a blind spot. This table lines them up so you can see which one covers for the others.
| Product | What it shows | What it hides | Best for |
|---|---|---|---|
| Text or zone forecast | Official wind and sea limits, warnings, a forecaster's plain-language read of the zone. | Fine detail. It averages a wide area and a broad time span, so it cannot tell you the hour or the precise location. | The official limits and the warnings. |
| GRIB file | Hour-by-hour wind, pressure, and waves on a fine grid, right along your route. | Human judgment. No fronts are drawn, and a missed squall line or rapid change can pass unflagged. | The hour-by-hour detail along your track. |
| Surface analysis chart | The highs, lows, and fronts, and the pressure pattern that drives the wind. | Exact numbers at a point. It shows the systems, not the precise wind speed at your waypoint. | Seeing a system coming and reading the big picture. |
| High-seas and offshore text | The official open-water picture and warnings once you leave coastal waters. | Local and coastal detail. It speaks to large ocean areas, not the lee of a headland. | Passages that leave the coastal zone. |
| Warnings and bulletins | A forecaster's flag that conditions have crossed a gale, storm, or special-warning threshold. | The detail of how the system develops, which you read from the chart and the GRIB. | Drawing the hard line you do not cross. |
What a GRIB file really is.
A GRIB file is a compact, gridded data format. It carries a weather model's raw forecast as a set of values on a grid of points: wind speed and direction, surface pressure, and wave height, written at a sequence of forecast hours. The format is built to be small, which is why it travels well over a slow satellite or radio connection at sea. The file itself is just numbers. Sailors view it in a phone app, computer software, or a chartplotter, which reads the grid and draws it as wind arrows, pressure lines, and wave shading you can scroll through hour by hour.
The key limit is easy to forget once the arrows are on the screen. A GRIB is model output with no human interpretation of it. No forecaster has looked at it, drawn a front on it, or added a warning. So it can miss things a trained meteorologist would normally catch: a front arriving early, a squall line, a low deepening fast. The numbers look the same whether the model has it right or wrong. Use the GRIB for what it is good at, the hour-by-hour detail along your route, and lean on the chart and the human-issued forecast for the judgment the GRIB does not contain.
Reading the inputs against your route.
WeatherWindow.AI is the synthesizer that this article describes. It reads your route against these forecast inputs and the surface analysis, projecting your position hour by hour and checking the wind, sea state, and systems at each point you will actually be. Then it returns one graded call for the passage, GO, CAUTION, MARGINAL, NO-GO, or DIVERT, with the reasoning behind it. You see the inputs behind the call, and you make the final decision.
Marine forecast questions, answered.
What is a GRIB file?
A GRIB file is a compact data file that carries a weather model's raw forecast on a grid of points. Each point holds values like wind, pressure, and waves at a sequence of forecast hours. It is the model output, packed small so it can travel over a slow connection at sea. You open it in an app or a chartplotter, which draws the numbers as wind arrows and pressure lines you can scroll through hour by hour.
What is the difference between a GRIB file and a marine forecast?
A GRIB file is raw model output with no human hand on it. It has good spatial and temporal resolution and updates with each model update, but no forecaster has checked it, so it can miss a front or a squall line that a person would flag. A text marine forecast is written by a forecaster. It has coarser resolution, is more conservative, and it carries official wind and sea limits and warnings. The GRIB gives you detail, and the text forecast gives you judgment. You want both.
What is a surface analysis chart?
A surface analysis chart is a map of the weather systems at sea level. It shows the highs and lows, the fronts between air masses, and the pressure lines that tell you where the wind is strong. It is the picture of why the wind is doing what it is doing. Reading it is the best way to see a system coming and to understand whether conditions are building or easing over your passage.
How often should I update the forecast on passage?
Roughly every twelve hours is a sensible habit on a normal passage, and always before any major decision, such as leaving shelter, rounding a cape, or pressing on past a bail-out point. New model updates land a few times a day, so a fresh download can change the picture. If the weather is active or a system is near, update more often.
Do I need to read GRIB files myself?
You can, and many sailors do, but you do not have to. The real skill is combining the products rather than reading any single one. You take the weather systems from the surface analysis charts, the hour-by-hour detail from the GRIBs, and the official limits and warnings from the text forecasts, then cross-check them against each other. A tool can do that combining for you, but the judgment of which call to make stays with you.
Which forecast should I trust if they disagree?
Lean on the human-issued forecast and the warnings for the limits, since a professional forecaster has assessed the situation and is being deliberately careful. Use the chart and the GRIB for timing and fine detail. When the products genuinely disagree, treat that disagreement as a signal of uncertainty rather than picking a winner. Plan for the worse of the two, keep a wider margin, and update sooner.
The models behind every GRIB and forecast. Read which weather model to trust →
References and further reading.
- UK Met Office: How to read synoptic weather charts
- Australian Bureau of Meteorology: The art of the chart: how to read a weather map
- Meteorological Service of Canada: GRIB format readme
- Royal Yachting Association: Weather and tides for boaters
- NOAA Ocean Prediction Center: About surface analysis charts
WeatherWindow.AI is a decision-support tool that assesses your intended passage. It is not a weather routing service. It does not replace official marine forecasts, a professional weather router, or your judgment as master.