How to read a marine forecast

A marine forecast is not a weather app with more numbers. It is a structured document, and once you know the structure you can read one in about thirty seconds and know whether your day is on or off. The trouble is that nobody explains the structure, so most people read the wind speed, ignore everything else, and get surprised.

The order the forecast is written in

Whether it comes from NOAA, the Met Office or a national weather service, a marine forecast follows the same logic: warnings first, then synopsis, then the period-by-period detail.

The warning line is the one you read first and the one people skip. A Small Craft Advisory, a Gale Warning, a Storm Warning: these are not decorations on top of the numbers, they are somebody with local knowledge telling you that the numbers add up to trouble.

The synopsis explains why the weather will do what it does — a front moving through, a ridge building, a low deepening. It matters because it tells you how much to trust the detail. A forecast built on a slow-moving ridge is far more reliable than one built on a fast-moving front whose timing could be off by six hours.

Wind: read three things, not one

• Direction. Always given as the direction the wind comes from. A southwest wind blows from the southwest. Direction decides whether your harbour is a shelter or a trap.
• Speed, and the range. "15 to 20 knots" means the sustained wind. Plan on the upper number, not the average of the two.
• Gusts. Gusts commonly run 30 to 40 percent above the sustained wind, and they are what catches you during a docking manoeuvre. If gusts are forecast to 30 knots, your day is a 30-knot day.

Also watch for a veering or backing wind in the text. A wind that shifts during the day can turn a sheltered anchorage into a lee shore while you are having lunch.

Seas: height alone tells you almost nothing

Forecast wave height is usually the significant wave height — the average of the highest third of the waves. That definition has a consequence people rarely absorb: individual waves will be higher. A rough working rule is that the largest wave in a few hours may be roughly twice the significant height.

The number that decides comfort is the period, the seconds between crests. Four feet at 10 seconds is a gentle roll. Four feet at 4 seconds is a wall of short, steep water that will stop a small boat. If the forecast separates wind waves from swell, read both: they can come from different directions and combine into a confused sea that is worse than either.

The line most people never read

Visibility. Fog does not make the sea rough, but it removes your primary navigation instrument — your eyes — in exactly the areas with the most traffic. A forecast with "visibility 1 nm or less in fog" deserves the same respect as one with 25 knots of wind.

How to actually use it

1. Read the warning line. If there is one, the decision is nearly made.
2. Check the trend, not the snapshot. Building or easing? Going out in improving conditions is a completely different day from going out in deteriorating ones.
3. Look at your return window. Afternoon sea breeze is predictable on most coasts: the trip home is often the hard part.
4. Compare two sources. When models agree, confidence is high. When they disagree, that disagreement is itself the forecast.
5. Look outside. The forecast covers a large area; your bay has its own opinion.

One last habit worth building: check the issue time. A forecast written twelve hours ago has been overtaken by reality. The most recent update is usually the only one worth acting on.

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