Chapter 7 / 12 · 6 min reading + practice
Assemble a route-specific information pack with official warnings, forecast areas, issue and validity times, observations and an onboard communications fallback.
A reliable weather routine starts with an information plan, not an application preference. Decide which official products cover each leg, what model guidance adds, which observations can test the present situation and how new warnings will reach the boat. A forecast retrieved before departure is only one version of a changing picture. Keep its provenance so another crew member can check it, rather than relying on a screenshot with an attractive colour scale.
Three information types, three different jobs
A forecast describes expected conditions for a stated place and time, with product-specific limits. An observation describes a measurement or report at a particular time and location; it is not a forecast and may be delayed, unrepresentative or faulty. A warning identifies an official hazardous-weather condition or expectation under the issuing service’s definitions. Read the warning text and instructions, not just an icon. Model guidance informs interpretation but is not automatically an official forecaster-reviewed bulletin.
Use a national meteorological or designated marine authority for official forecasts and warnings. WMO’s WWMIWS portal indexes high-seas information by METAREA and identifies the responsible services. It also warns that internet delivery can be interrupted or delayed and points readers to the originating national authority. A global portal is therefore an entry point, not proof that your specific coastal passage has been completely checked.
Select areas by the route, not by a town name
Map the start, offshore legs, exposed headlands, approaches and alternatives against the official boundaries. NOAA distinguishes coastal, offshore and high-seas products and provides zone-based marine listings. The UK Shipping Forecast uses named sea areas; an inshore forecast may answer a different question. Do not transplant one service’s naming system to another country, and do not assume a broad ocean forecast resolves a harbour entrance. If the route sits on a boundary, read both neighbouring areas and the local authority’s guidance.
Preserve the product identifier, direct source address, responsible organisation and coverage description in the briefing. A link to a ministry’s homepage is less useful than the current marine product. However, a direct link can change or serve a cached file, so test it and retain a fallback entry page. Keep coastal weather warnings separate from navigation warnings, port closures and other maritime safety information: they can interact, but one list does not replace the others.
Four clocks to keep apart
- Issue time: when the bulletin was released, or when an observation report was published.
- Valid time or interval: when its conditions apply; an instantaneous model value is not necessarily a whole-period average.
- Model reference time: the initial state from which a numerical forecast was generated, not the download time.
- Received time: when your boat or service obtained the data, which may expose a delivery delay.
Store the date, year and explicit UTC or documented local basis. Retain original wording when a bulletin says later, occasionally or at first; translate those terms using that service’s explanations, not arbitrary universal hour ranges. A recent download can contain an old issue. An issue released recently can also begin after your immediate departure. Both checks matter. Mark unknown times as unknown rather than inventing a timestamp.
GRIB is a format, not a complete marine briefing
A GRIB file packages selected numerical fields and their metadata. Your request decides geographical extent, variables, steps and sometimes resolution. The file does not acquire missing warnings, fronts, squall detail or local expertise merely because the display animates. Check the producing centre and model, reference run, variable definition, valid steps, units and land/sea representation. Do not mistake a smoothly interpolated map for measured detail between grid points. Keep official text products alongside the model files, including a lightweight saved copy for low-bandwidth conditions.
Receiving information while underway
GMDSS is the international framework for maritime distress and safety communications, including weather-related maritime safety information. Depending on the area and suitable equipment, weather information is distributed through NAVTEX, satellite EGC services such as SafetyNET or SafetyCast, and relevant radio services. Coastal VHF broadcasts can be useful where covered. A phone application or ordinary internet connection is not the same as a configured maritime safety receiver. Check actual station coverage, schedules, receiver settings, power and licensing with current official guidance.
Do not imply every leisure yacht has the same statutory carriage requirements as a SOLAS ship. Instead, identify your vessel’s applicable rules and build a practical redundant reception plan. Test reception before sailing, preserve the correct forecast areas or station selections and assign someone to notice missing updates. If a channel fails, the briefing should show its last successful receipt, the resulting uncertainty and the alternative channel—not silently continue to label old data current.
Worked information-pack exercise
This route and all times are hypothetical. A six-hour day passage leaves at 07:30 UTC and is expected to arrive at 13:30. It crosses fictional coastal zones A and B. At 07:10 the crew downloads a bulletin issued 06:00, valid 06:00–12:00, plus an observation measured 06:50. The observation helps assess the present situation; it does not extend the bulletin’s validity to arrival. The plan needs the following period’s forecast, both route zones and continued warning reception.
At 10:40 an invented official warning is issued for zone B, valid 11:00–14:00. Re-open the route timeline: when does the boat reach B, which escape or waiting options remain, and is the original estimate still realistic? The old briefing cannot override the new warning. The crew records the warning’s source and issue, checks its coverage and meaning, obtains the updated forecast, and reassesses with vessel-specific constraints. This exercise is about disciplined information use, not a numerical rule deciding whether to continue.
The pack to retain and mistakes to avoid
- List all route and alternative areas with direct official forecast and warning links.
- Record product type, source, issue, validity, reference run where relevant and last receipt.
- Add suitable observations with measurement times and representativeness notes.
- Document known gaps: unavailable product, expired interval, unclear boundary or failed channel.
- Schedule the next update around the next decision point, not merely a convenient clock time.
- Keep a concise offline summary, original official wording and a tested onboard fallback.
A blank warning panel can mean no retrieved warning, a service failure, incomplete coverage or a genuinely clear checked product. Distinguish these states; none proves the sea is safe. An AI summary must preserve sources, dates, caveats and unresolved gaps. It may make a long bulletin easier to understand, but it must not invent a bulletin, conceal a warning or upgrade a missing observation into evidence.
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1. Show explanation
Warnings not checked or unavailable, with the failed source and need for an official recheck made explicit. A failed request provides no evidence that warnings are absent. Warning coverage and retrieval state must remain distinct from numerical model availability. Show the gap and restore the official check rather than converting missing data into reassurance.
2. Show explanation
Applicable information for the later part of the passage, plus ongoing warning updates for the route zones. The bulletin's valid interval ends before the planned arrival. Neither receipt time nor an observation extends that interval. Obtain the following period's applicable forecast and continue checking warnings through the relevant decision points.
3. Show explanation
It provides evidence about that station at 06:50, subject to age, exposure and representativeness limits. Measurement, publication and receipt times describe different events. The observation helps test the present weather picture but is not an offshore forecast. Check whether the station's location and exposure meaningfully represent the part of the route being assessed.
Check your understanding
First explain your answer in your own words, then open the explanation. These are original Sailing Weather exercises, not official exam questions.
1. Can a fresh download prove a forecast is current?
No. Check original issue, validity, model reference where applicable and delivery delay.
2. Does a 06:50 observation extend a bulletin valid only until 12:00 to a 13:30 arrival?
No. An observation describes its own time and place; obtain forecast coverage for arrival.
3. A global GRIB animation shows light wind. Where are official warnings?
In the responsible authority's relevant products and appropriate onboard reception channels; do not assume the selected GRIB contains them.
4. What should an AI report say when a warning feed cannot be retrieved?
Coverage is unavailable or unchecked, with last-receipt information if known. It must not substitute 'no warnings'.
Sources and verification scope
- WMO/IMO: Worldwide Met-Ocean Information and Warning ServiceMETAREA information, originating authorities and internet-delivery limitations.
- NOAA/NWS: marine text forecasts by zoneArea-specific marine products and distinctions among coastal, offshore and high-seas information.
- NOAA/NWS: GMDSS disseminationNAVTEX, SafetyNET/SafetyCast and maritime safety dissemination; equipment and area coverage matter.
- Met Office: Shipping Forecast and gale warningsNamed sea areas, separate issue/validity information and warning not to rely solely on internet delivery.
- ECMWF ecCodes: GRIB time metadataReference and validity time are metadata fields, not the user's retrieval timestamp.