In short
- Check the issue time and the period first, then the zone or area. A forecast is only about the water and the hours it names.
- Wind direction is where the wind comes from. Speeds are in knots, and gusts typically run 20 to 40 percent above the sustained wind, more in showers.
- Seas are significant wave height: the average of the highest third. Expect the odd wave well above it, and read the period next to the height.
- A barometer falling more than 3.5 hPa in three hours means wind is on its way, whatever the sky looks like.
When and where it applies
Every marine forecast starts with two things people skip: when it was issued and what it covers. Read them first. A forecast saved last night may describe a different set of hours from the one issued this morning, and the wind in the next zone along the coast can be a different story.
In the US, the National Weather Service writes a coastal waters forecast for zones that generally reach out to 20 nautical miles, usually issued four times a day and updated when things change. Beyond that come the offshore waters and high seas forecasts. Each zone has its own name and code, and the text runs period by period: today, tonight, tomorrow, and so on out to several days.
In the UK, the Met Office inshore waters forecast covers stretches of coast out to 12 nautical miles, four times a day, with 24 hours of detail and an outlook for the next 24. The shipping forecast covers the 31 sea areas around the British Isles, further offshore. The inshore forecast is the one most coastal trips need, but if your route leaves its range, read the shipping forecast for the area too.
Point forecasts and apps give you a single spot. They are useful for detail, but read the area forecast first so you know the story the forecaster is telling for the whole stretch of water.
Wind: direction, speed and gusts
Direction is where the wind comes from. A southwesterly blows from the southwest. Forecasts use eight or sixteen compass points, and they describe changes with two words: the wind veers when it shifts clockwise, from south to southwest to west, and backs when it shifts anticlockwise. A veer behind a cold front is a classic sign that the front has passed.
Speed is in knots in US forecasts, written as a range: "SW winds 15 to 20 kt". UK forecasts use the Beaufort force: "southwest 5 or 6". The table below converts between them.
Sustained wind is an average; gusts are short peaks. A gust lasts a few seconds. Over open water gusts typically run 20 to 40 percent above the mean, and more in showery or unstable air or close to high land. When the forecast says "gusts up to 25 kt", that is the number your sails and your crew will feel in the puffs, so plan around it as well as the range.
| Force | Knots | What the sea looks like |
|---|---|---|
| 3 Gentle breeze | 7 to 10 | Large wavelets, crests begin to break |
| 4 Moderate breeze | 11 to 16 | Small waves, fairly frequent white horses |
| 5 Fresh breeze | 17 to 21 | Moderate waves, many white horses, some spray |
| 6 Strong breeze | 22 to 27 | Large waves form, white foam crests everywhere |
| 7 Near gale | 28 to 33 | Sea heaps up, foam blown in streaks |
| 8 Gale | 34 to 40 | Moderately high waves, crests break into spindrift |
| 9 Severe gale | 41 to 47 | High waves, dense streaks of foam, spray cuts visibility |
Seas: height, period and direction
The sea height in a forecast is the significant wave height: the average height of the highest third of the waves. It is close to what an experienced eye reports as "the sea", which is why it was chosen. It is not the biggest wave. About one wave in ten is roughly a quarter higher, and over a few hours of sailing, the odd wave can approach twice the forecast figure.
- 1Significant wave height, about 1.5 m here: the average of the highest third, shown in dark blue. This is the number in the forecast.
- 2The biggest wave in this run of 64 waves, about one and a half times the significant height.
- 3Most waves are smaller than the forecast figure, which is why a sea can feel easier than forecast until a set arrives.
Period is the time between crests, in seconds. With the height, it tells you how steep the sea is: 1.5 metres at 12 seconds is a long, easy swell, while 1.5 metres at 5 seconds is a short, hard chop. Our guide to wave height and wave period works through it.
Direction for waves, like wind, is where they come from. Many NWS offices now add a wave detail line that splits the sea into its parts, such as "S 3 ft at 6 seconds and SE 2 ft at 9 seconds". That is a local wind sea and a swell from a different direction, and the split matters more than the total. See swell vs wind waves for how they add up.
UK forecasts describe the sea in words. The scale is fixed, so it converts straight to metres:
| Sea state | Significant wave height |
|---|---|
| Smooth | Up to 0.5 m |
| Slight | 0.5 to 1.25 m |
| Moderate sea | 1.25 to 2.5 m |
| Rough | 2.5 to 4 m |
| Very rough | 4 to 6 m |
| High | 6 to 9 m |
Weather and visibility
The weather word, such as showers, rain, thundery showers or fog patches, tells you what to expect from the sky. Showers bring gusts and short drops in visibility; thundery showers bring squalls that can double the wind for a few minutes. Fog patches mean visibility will vary a lot over short distances.
UK forecasts grade visibility with four words, and US forecasts give it in nautical miles when it matters:
- Good: more than 5 nautical miles.
- Moderate: 2 to 5 nautical miles.
- Poor: 1,000 metres to 2 nautical miles.
- Very poor or fog: under 1,000 metres.
Fog at sea usually comes from warm, moist air moving over colder water. It can sit offshore all day in light winds and roll in on a sea breeze, so a clear harbour in the morning says little about the headland at noon.
Pressure and its trend
The barometer reading on its own says less than its trend. A low at 1005 hPa that is filling is a fading threat; a high at 1020 hPa that is falling fast is not. The Met Office describes the change over three hours with fixed words, and they are worth learning because they map directly onto how soon the wind arrives.
- 1Steady: less than 0.1 hPa in three hours.
- 2Falling slowly: 0.1 to 1.5 hPa in three hours.
- 3Falling: 1.6 to 3.5 hPa in three hours.
- 4Falling quickly: 3.6 to 6 hPa in three hours. More than 6 is falling very rapidly.
A rule of thumb that has served sailors for a long time: a fall of more than 3.5 hPa in three hours usually means strong wind within the next 12 hours, and a fall of more than 6 hPa means it is close. When the barometer and the forecast disagree, the barometer is telling you something the forecast has not caught yet.
Timing words and model agreement
UK forecasts and gale warnings use three timing words with exact meanings, counted from the time of issue:
- Imminent: within 6 hours.
- Soon: in 6 to 12 hours.
- Later: more than 12 hours away.
US forecasts give each change a period, such as "this afternoon" or "after midnight". "At times" or "occasionally" means the condition comes and goes, not that it holds for the whole period.
Confidence is the other half of a forecast. Forecasters lean on several computer models, and so can you. The American GFS, the German ICON and the European ECMWF model are run separately, from different starting analyses. When all three put the front through at the same hour with the same wind, the forecast is on firm ground. When one brings it in at dawn and another at dusk, the timing is uncertain, and a trip that depends on that timing deserves another look when the next run comes in.
Worked example: two real-style forecasts
A US coastal waters forecast, one period
"This afternoon: SW winds 15 to 20 kt with gusts up to 25 kt. Seas 3 to 4 ft. Wave detail: S 3 ft at 6 seconds and SE 2 ft at 9 seconds. A chance of showers and thunderstorms."
Read it as: a fresh southwesterly, force 4 to 5, with puffs to force 6. The sea is mostly a short wind sea from the south, 6 seconds between crests, with a smaller southeasterly swell underneath, so expect a lumpy, slightly confused motion. Showers and thunderstorms bring squalls, so the gusts could be higher near cells, and visibility will drop briefly in rain.
A UK inshore waters forecast, one area
"Southwest 4 or 5, increasing 6 at times later. Slight or moderate, occasionally rough later in west. Showers. Good, occasionally moderate."
Read it as: 11 to 21 knots from the southwest, with spells of 22 to 27 knots more than 12 hours from issue. Seas 0.5 to 2.5 metres, reaching 2.5 to 4 metres at times in the west of the area later on. Showers, with visibility over 5 miles, dropping to 2 to 5 miles in them. If your passage runs west and finishes late, the last leg meets the worst of it.
That last step is the one that matters: put the forecast against the hours you'll be in each place. Our guide to finding a weather window shows how to do it for a whole passage.
