Reading a marine forecast: synopsis, sea areas, sea state and warnings

A marine forecast is a weather service's official forecast for defined sea areas. It opens with the general synopsis, then gives for each sea area the wind in Beaufort with direction and change, visibility and weather and the sea state, and closes with the outlook. Its strong wind and gale warnings take precedence over any app.

How is a marine forecast structured?

Almost always the same way: header, warnings, general synopsis, forecast for each sea area and outlook. Once you know the order, you will find your area quickly.

One area from a DWD forecast reads like this (translated from German):

• German Bight, wind: south to southwest 4, occasionally around 5, veering west to northwest, later thunderstorm gusts 8.
• Visibility/weather: later heavy rain and thunderstorms with poor visibility.
• Sea state: northern part increasing 2 metres.

In plain terms: wind from south to southwest at 11 to 16 kn, at times about 17 to 21 kn, veering through west to northwest. Later, thunderstorms bring gusts of 34 to 40 kn and poor visibility, and in the northern part the sea builds to 2 m.

What do the terms in a marine forecast mean?

The wording is terse and always the same. Wind is given as the direction it blows from, with its strength in Beaufort, so as mean wind; gusts are stronger.

For visibility the Shipping Forecast uses fixed thresholds: good more than 5 nm, moderate 2 to 5 nm, poor 1,000 m to 2 nm, very poor less than 1,000 m. The DWD speaks of good, moderate, poor and very poor visibility. The full scale with knots is in the guide Wind force on the Beaufort scale.

How is sea state given?

The DWD gives it in metres, the Shipping Forecast in words. What is meant is the significant wave height, the average of the highest third of the waves; individual waves get considerably higher.

How wind sea, swell and period interact is explained in the guide Sea state and waves.

How do the DWD, Shipping Forecast and MET Norway work?

All three follow the same pattern, each with its own areas and wording.

DWD

The marine weather service of the DWD (Germany's national weather service) in Hamburg issues a forecast for the North Sea and the Baltic, with sea areas from the Southwestern North Sea and the German Bight via the Skagerrak, Kattegat and the Belts and Sound to the Gulf of Finland. On top of that come a forecast for the German coast, with sections such as the Elbe estuary or Flensburg to Fehmarn, and one for the Mediterranean that names the areas with strong winds, gales or hurricane-force winds. Wind in Beaufort, pressure in hectopascals, times in UTC. Strong wind warnings cover force 6 and 7, gale warnings force 8 and above.

Met Office Shipping Forecast

Four times a day, at 23:00, 05:00, 11:00 and 17:00 UTC, for the following 24 hours and 31 sea areas around the British Isles, from Viking and German Bight to Fastnet, Shannon and Southeast Iceland. First the gale warnings (force 8 and above), then the general synopsis, then wind, sea state, weather and visibility for each area. Timing words: imminent means within 6 hours, soon in 6 to 12 hours, later in more than 12 hours.

MET Norway

Forecasts for Norwegian coastal waters and the sea areas up to Svalbard, Jan Mayen and into the Denmark Strait, in Norwegian and English. Careful with the wording: “liten kuling” is already force 6, “stiv kuling” 7, “sterk kuling” 8, “liten storm” 9 and “full storm” 10.

Which sea areas apply to my trip?

All the areas your route touches, plus the area to windward that the weather comes from. The services' maps show the boundaries.

Near national borders, forecasts from two services often apply, each with its own names: the DWD's Deutsche Bucht is German Bight in the Shipping Forecast, and its Fischer is Fisher there. If your route runs through such waters, read both.

Where do I get the marine forecast?

On board via VHF, NAVTEX and shortwave, and online when in harbour.

The internet is not part of the official maritime safety broadcasts, as the Met Office itself points out: at sea, NAVTEX and VHF are the reliable route.

Why is the marine forecast better than an app when it comes to warnings?

Because it is written by meteorologists who weigh up several models, observations and local effects, and because its warnings are official. An app, by contrast, usually shows the output of one model on a grid.

Models smooth things out: thunderstorm gusts, acceleration around headlands and the full strength of local winds often come through too weak. The forecast names them, even if they only occur in places. The best approach is to use both: the model for how conditions develop along your course, the forecast for the overall situation and warnings. How the models differ is covered in the guide Weather models.

How does Sailseer show marine forecasts?

For every route and every point on the map, Sailseer shows the official marine forecasts for the areas the route touches within a 20 nm margin, verbatim with service, time of issue, language and link: DWD, KNMI, MET Norway, AEMET, IMO, Met Office, Météo-France, HNMS, DHMZ, DMI for Greenland and more, all the way to the Indian Ocean, with German-language forecasts listed first. Thunderstorm gusts from the DWD forecast feed into the rating, and a gale warning in the area sets a departure to zero points. New official warnings for a harbour are reported by the warning alert by email or push.

Sailseer is a planning aid and not for navigation. The service's own forecast is authoritative, and always the latest one.