Learn how commercial ships receive weather forecasts, storm warnings and maritime safety information through NAVTEX, GMDSS satellites, radiofax, GRIB files and professional weather-routeing services.
Far from land, a ship cannot simply look at a weather app and hope for the best. A commercial vessel may be several days from the nearest port, with limited internet access and a storm developing across its planned route. The bridge team needs weather information that is reliable, up to date and suitable for decisions at sea.
Ships therefore use several systems together. Close to the coast, NAVTEX automatically receives weather and navigational warnings. On ocean passages, satellite systems deliver warnings and forecasts over much larger areas. Ships may also receive radiofax charts, voice broadcasts, email forecasts, GRIB data and advice from professional weather-routeing services.
The important point is that no single screen tells the whole story. Safe navigation depends on receiving the information, checking its time and area, comparing different sources and then applying professional judgement.
Weather information is part of maritime safety
Weather affects almost every part of a voyage. Strong winds and high waves can reduce speed, damage cargo, cause heavy rolling and pitching, and place dangerous loads on the hull. Fog reduces visibility. Ice can close routes or damage the ship. Tropical cyclones may require a large route alteration several days in advance.
Weather also affects fuel consumption and the machinery plant. A ship pushing into heavy head seas may need more power but make less speed. Propeller racing can increase, engine load may fluctuate, and seawater can reach deck equipment and ventilation openings. On some vessels, cargo temperature, lashing loads, stability, slamming, parametric rolling or liquefaction risks must also be considered.
For this reason, meteorological warnings and forecasts form part of Maritime Safety Information, usually shortened to MSI. MSI includes navigational warnings, meteorological warnings, weather forecasts and other urgent safety messages broadcast to ships.
Who prepares marine weather forecasts?
Marine forecasts are normally prepared by national meteorological and hydrological services or another national authority appointed for that work. These organisations use information from satellites, weather buoys, coastal stations, aircraft, radar, ships and numerical forecast models. Trained meteorologists analyse the data and issue official forecasts and warnings.
The World Meteorological Organization and the International Maritime Organization coordinate the global system through the Worldwide Met-Ocean Information and Warning Service, or WWMIWS. The oceans are divided into 21 METAREAs. Each METAREA has a coordinator responsible for organising the provision of marine forecasts and warnings for shipping in that region.
The METAREA system operates within the Global Maritime Distress and Safety System, better known as GMDSS. This gives ships a structured way to receive official safety information even when normal commercial communications are unavailable.
The main ways ships receive weather information
A modern commercial ship may use all of the following:
- NAVTEX for coastal Maritime Safety Information.
- Satellite Enhanced Group Call services for wider sea areas.
- HF radiofacsimile, commonly called weatherfax or radiofax.
- VHF, MF or HF voice weather broadcasts.
- Satellite internet, email and company communications.
- GRIB weather files and electronic weather charts.
- Professional weather-routeing services.
- Onboard instruments and direct visual observations.
These systems overlap, but they are not identical. Some are mandatory safety services. Others are commercial or supplementary tools. A prudent bridge team knows which is which.
What is NAVTEX?
NAVTEX is an automatic text-broadcast service designed to provide ships with Maritime Safety Information in coastal waters. A shore station transmits messages by radio, and the ship’s dedicated NAVTEX receiver displays, stores or prints them automatically.
The receiver can work without the bridge team requesting each forecast. When it is powered, correctly configured and within the station’s service area, it listens for scheduled transmissions and records the relevant messages.
According to the IMO NAVTEX Manual, International NAVTEX provides navigational warnings, meteorological warnings, meteorological forecasts and other urgent safety-related information. It is an important part of GMDSS.
NAVTEX frequencies
The main frequencies are:
| Frequency | Service | Normal language |
|---|---|---|
| 518 kHz | International NAVTEX | English |
| 490 kHz | National NAVTEX | Language selected by the national administration |
| 4209.5 kHz | National NAVTEX | Language selected by the national administration |
Some countries may also use other nationally assigned frequencies.
International stations share 518 kHz using coordinated time slots. Each transmitter has an identification letter and a scheduled broadcast period. This prevents nearby stations from transmitting over one another. The ship’s receiver can be set to accept the stations relevant to its voyage.
NAVTEX coverage is not exactly the same everywhere. It depends on the transmitter, the defined service area, local radio conditions and the surrounding geography. It should therefore be treated as a coastal-area service, not as a guaranteed mid-ocean system.
Understanding NAVTEX message codes
Each NAVTEX message contains a technical identity. Part of that identity tells the receiver which station sent the message and what type of information it contains.
Some useful subject letters are:
| Subject letter | Information type |
| A | Navigational warnings |
| B | Meteorological warnings |
| C | Ice reports |
| D | Search and rescue information, piracy warnings, tsunami warnings and certain other natural hazards |
| E | Meteorological forecasts |
On compliant receivers, essential categories such as navigational warnings, meteorological warnings and search-and-rescue information cannot simply be switched off. This reduces the risk of a ship missing critical safety information.
Meteorological warnings are given a higher priority than routine forecasts. A normal forecast may arrive in the scheduled broadcast, while an urgent warning can require faster transmission and repetition.
What the officer on watch should do with a NAVTEX message
Receiving the message is only the first step. The officer should:
- Read the complete message, not only the heading.
- Check the issuing station, date and UTC time.
- Confirm the geographical area and validity period.
- Identify whether it is a forecast, warning or cancellation.
- Plot important positions, fronts, cyclone tracks or warning boundaries where relevant.
- Compare the information with the ship’s route and expected position.
- Inform the Master according to standing orders.
- Keep or record the message as required by the ship’s procedures.
A forecast for the correct sea area but the wrong validity time can be just as misleading as a forecast for the wrong area.
How do ships receive forecasts in the open ocean?
NAVTEX is valuable near the coast, but an ocean-going ship quickly sails beyond the service areas of coastal transmitters. Wider-area information is then normally received through satellite-based Enhanced Group Call, or EGC, services recognised for GMDSS.
The two best-known international EGC services are:
- Inmarsat SafetyNET
- Iridium SafetyCast
These services automatically broadcast Maritime Safety Information and search-and-rescue information to ships in selected geographical areas. Messages may include meteorological warnings, forecasts, tropical cyclone information, navigational warnings and urgent safety notices.
The ship does not need to request every warning individually. The terminal receives broadcasts addressed to its present ocean region or another selected area. Correct terminal setup is therefore essential. The equipment must have an accurate ship position and the relevant NAVAREA and METAREA selections for the voyage.
Satellite MSI is not simply ordinary internet weather. It is part of a coordinated safety system. Commercial email or broadband may fail because of an equipment problem, subscription issue or network interruption, while the GMDSS installation is arranged and maintained for safety communications.
IMO guidance also recognises HF narrow-band direct printing in certain operating areas. The correct receiving arrangement depends on the services available throughout the ship’s intended voyage.
Radiofax: weather charts sent by radio
Radiofacsimile, often called weatherfax or radiofax, sends graphical weather charts over HF radio. A receiver and decoder reproduce the chart on a printer or computer. Radiofax products may include:
- Surface-pressure analysis charts.
- Forecast pressure charts.
- Wind and wave charts.
- Tropical weather information.
- Ice charts.
- Satellite images.
These charts help officers see the position of pressure systems, fronts, troughs and areas of strong wind or high waves. This visual picture is useful when comparing several possible routes.
Radiofax remains valuable because it can receive large-area charts without normal internet access. However, reception quality can be affected by frequency choice, time of day, atmospheric conditions, antenna condition and radio interference. The bridge team needs the correct broadcast schedule and frequencies for the region.
Voice weather broadcasts
Coast radio stations and meteorological services may transmit marine weather by VHF, MF or HF voice. The available service depends on the country and sea area.
VHF weather broadcasts are mainly useful near the coast because VHF is largely limited by line of sight. HF radio can cover much greater distances, although reception varies with frequency and atmospheric propagation.
Voice broadcasts are useful, but they require an active listening watch and careful note-taking. A misunderstood position or wind force can change the meaning of a warning. Officers should use the published station lists, frequencies and schedules carried on board.
Satellite internet, email and commercial weather services
Most modern merchant ships have some form of satellite communication. The bridge or company may receive weather charts, text forecasts and routeing advice by email or through an online platform.
Commercial weather providers can supply detailed information tailored to the ship, including:
- Forecast wind, waves, swell and currents.
- Tropical cyclone tracks and avoidance guidance.
- Recommended routes and waypoints.
- Predicted ship speed and fuel consumption.
- Heavy-weather alerts.
- Estimated arrival times for alternative routes.
These services are powerful planning tools, but they do not remove the Master’s responsibility. A shore routeing recommendation is based on forecast data and a model of the ship’s performance. The Master still has to consider the actual vessel, cargo, stability, machinery condition, traffic, charted dangers and conditions observed on board.
What are GRIB files?
GRIB means gridded binary. A GRIB file is a compact digital file containing forecast-model data arranged on a geographical grid. Navigation or weather software can display the information as wind arrows, pressure lines, wave heights, rainfall, currents and other layers.
GRIB files are popular at sea because they are small and can be downloaded over a limited satellite connection. They also allow officers to move forward through forecast times and compare the ship’s route with expected conditions.
However, a GRIB image can look more certain than the forecast really is. Some GRIB products are direct model output without a marine forecaster correcting the result or adding an official warning. Resolution may also hide local effects, small but intense systems or the highest individual waves.
GRIB data should therefore be used together with official meteorological warnings, written forecasts, analysed weather charts and other available information. It should not be treated as a replacement for GMDSS Maritime Safety Information.
Onboard observations still matter
A ship does not only receive weather information. It also observes the atmosphere and sea around it.
Bridge instruments may show:
- Atmospheric pressure and pressure trend.
- True and relative wind speed and direction.
- Air and sea temperature.
- Visibility.
- Rainfall or nearby squalls on radar.
- Wave and swell direction estimated by observation.
The barometer is especially useful. A falling pressure trend may confirm that a low-pressure system is approaching. A rapid or unexpected change should make the officer compare the actual conditions with the latest forecast.
Weather radar can show nearby rain, squalls and thunderstorms, but it is not a long-range ocean forecast. Likewise, the ship’s wind sensor reports conditions at the vessel now; it does not show what will happen 500 nautical miles ahead.
Some ships also participate in voluntary observing programmes and send weather reports to meteorological organisations. These ship observations help improve the data available to forecasters and numerical models, especially in ocean areas with few fixed observing stations.
What information is contained in a marine forecast?
A well-structured high-seas forecast commonly includes:
- Warnings – for example gale, storm, hurricane-force wind, tropical cyclone, heavy freezing spray or very high waves.
- Synopsis – the present or expected positions and movements of major pressure systems and fronts.
- Forecast – expected wind, sea and weather conditions for defined areas and times.
Depending on the region and season, the message may also include:
- Wind direction and force or speed.
- Significant wave height.
- Swell direction, period and height.
- Visibility restrictions.
- Fog, thunderstorms or heavy precipitation.
- Sea ice and iceberg information.
- Freezing spray.
- Tropical cyclone position, movement and forecast track.
The words issued, valid, from, until, moving, and forecast position are important. A cyclone’s present position and its 24-hour forecast position are not interchangeable.
How weather forecasts support voyage planning
Weather is considered before departure and throughout the voyage.
Before sailing
The bridge team studies seasonal conditions, expected winds and waves, tropical cyclone activity, ice limits, currents and possible ports of refuge. The passage plan should consider known weather dangers and the ship’s operational limits.
During the ocean passage
Forecasts are updated regularly. The route may be adjusted to avoid the strongest part of a system, reduce head seas or place the ship in a safer part of a tropical cyclone. Sometimes the safest decision is to reduce speed, wait, change the sequence of waypoints or remain in sheltered water.
During each watch
The officer compares the forecast with actual pressure, wind, sea state, cloud and visibility. New warnings are checked against the planned track. Significant changes are reported to the Master.
Weather routeing is therefore a continuous process, not a document completed before departure and forgotten.
A simple heavy-weather example
Imagine a container ship crossing the North Atlantic. The passage plan passes south of a developing low-pressure system.
During the evening watch, the satellite terminal receives a new gale and high-wave warning. The latest forecast shows that the low is deepening more quickly and moving farther south than predicted. A new GRIB file shows a band of strong head winds crossing the route in 30 hours. The barometer on board has also begun to fall faster.
The officer does not rely on the colourful GRIB display alone. The officer reads the official warning, plots the forecast system, checks the valid time and calls the Master. The Master compares the warning with weatherfax charts, routeing advice, cargo limits and the ship’s expected position.
The ship may alter course early, reduce speed or pass on another side of the system. Deck cargo, anchors, openings and loose equipment may be checked. The engine room is informed so that machinery and tank arrangements can be prepared for heavy rolling and fluctuating propulsion load.
An early alteration may add distance, but it can reduce damage, fuel waste and risk. Waiting until the ship reaches the storm leaves fewer safe choices.
Common misunderstandings
| Misunderstanding | Correct explanation |
| NAVTEX works everywhere in the world | NAVTEX is mainly a coastal service with defined transmitter service areas. |
| Satellite internet replaces GMDSS weather reception | Ordinary internet is useful, but GMDSS MSI uses recognised and coordinated safety services. |
| A GRIB file is always an official weather warning | GRIB data may be direct numerical-model output and must be checked against official warnings and forecasts. |
| Weather radar shows the weather for the next several days | Ship radar mainly detects nearby precipitation and squalls. |
| Once a passage plan is approved, the route is fixed | The route must be monitored and amended when safety requires it. |
| One forecast source is enough | Good bridge practice compares official MSI, charts, model data, routeing advice and actual observations. |
Practical checklist for cadets and junior officers
- Know which METAREA and NAVAREA the ship is sailing in.
- Confirm that the NAVTEX receiver is powered and the correct stations are selected.
- Do not suppress essential message categories.
- Check the satellite MSI terminal’s position and area settings.
- Read the complete warning, including issue time and validity.
- Plot important forecast positions or warning boundaries when required.
- Compare forecasts with the ship’s route and expected positions.
- Compare GRIB data with official text warnings and analysed charts.
- Monitor the barometer, wind, sea, swell, cloud and visibility.
- Follow the Master’s standing orders for reporting weather changes.
- Record or file received warnings according to the ship’s procedures.
- Never assume that silence from one receiver means there is no warning.
Frequently asked questions
Does NAVTEX work in the middle of the ocean?
Normally, NAVTEX should be regarded as a coastal-area service. Actual coverage depends on the station and published service area. Ocean-going ships use appropriate satellite EGC or other recognised services for wider coverage.
Can ships use normal internet weather websites?
Yes. Internet products can be very useful when the connection is available. They are supplementary sources and should be used together with the ship’s required GMDSS arrangements and official maritime warnings.
What is the difference between a weather warning and a forecast?
A forecast describes expected conditions over a stated area and time. A warning highlights conditions that may create a serious danger, such as gale-force winds, a tropical cyclone, heavy freezing spray or very high waves.
Who is responsible for checking the weather on a ship?
The officer of the watch normally monitors incoming information and actual conditions. The Master remains responsible for the vessel’s safe navigation and for major voyage decisions. Company procedures and the Master’s standing orders define when the Master must be called.
Is weather-routeing advice compulsory?
Routeing advice may be required by company or charter arrangements, but it does not replace the Master’s judgement or legal responsibility. Safety remains the first consideration.
Final thought
Ships receive weather forecasts through a layered system. NAVTEX provides automatic coastal warnings and forecasts. Satellite EGC services carry Maritime Safety Information across ocean regions. Radiofax, voice broadcasts, internet products, GRIB files and professional routeing services add more detail. Onboard instruments show whether the real weather is developing as expected.
Technology brings the information to the bridge, but people turn it into a safe decision. The best bridge teams do not ask only, “What does the forecast say?” They also ask, “Is it valid for our position and time, do other sources agree, and what action should we take now?”
References and further reading
- International Maritime Organization, NAVTEX Manual, 2023 Edition — MSC.1/Circ.1403/Rev.2: https://wwwcdn.imo.org/localresources/en/OurWork/Safety/Documents/Documents%20relevant%20to%20GMDSS/MSC.1-Circ.1403-Rev.2.pdf
- World Meteorological Organization and International Maritime Organization, Worldwide Met-Ocean Information and Warning Service, including the 21 METAREAs: https://wwmiws.wmo.int/
- International Maritime Organization, IMO circulars related to the GMDSS, including SafetyNET, SafetyCast, NAVTEX and MSI reception guidance: https://www.imo.org/en/ourwork/safety/pages/imo-circulars-related-to-the-gmdss.aspx
- International Maritime Organization, Guidance for the reception of MSI and SAR-related information — MSC.1/Circ.1645: https://wwwcdn.imo.org/localresources/en/OurWork/Safety/Documents/Documents%20relevant%20to%20GMDSS/MSC.1-Circ.1645.pdf
- National Oceanic and Atmospheric Administration / National Weather Service, Marine Radiofax Charts: https://www.weather.gov/marine/radiofax_charts
- National Weather Service, Dissemination of Marine Weather: https://www.weather.gov/marine/nws_dissemination
- Iridium, Iridium GMDSS and SafetyCast: https://www.iridium.com/services/iridium-gmdss