Marine Weather Routing

Weather is the variable that most often turns a well-found boat's passage into a bad experience. It is also the area where recreational capability has changed most in the last decade: a cruising yacht can now carry the same model data a professional navigator uses.

That creates a new failure mode. Model data looks precise. It is not the same as being right.

#The three layers of marine weather information

#Official forecasts and warnings

For Australian waters, the Bureau of Meteorology's coastal waters forecasts, high seas forecasts, and wind and gale warnings are the authoritative products. They carry forecaster judgement — a human has looked at the models, weighted them, and decided what to emphasise.

Warnings in particular should be treated as decision triggers, not as background information. A strong wind warning that appears after you planned the passage is a reason to revisit the plan.

#Synoptic and prognosis charts

Pressure charts show the systems behind the numbers: highs, lows, troughs, fronts and their expected movement. This is the layer most recreational skippers skip, and it is the layer that builds an actual mental model.

If you can see that a front will cross the coast around midnight and that the wind will back and strengthen behind it, a wind arrow on a GRIB display stops being a mystery.

#Model data (GRIB)

GRIB files are compact gridded model output — wind, pressure, rain, waves, sometimes surface current — at regular points and time steps. They are small, download easily over satellite, and display directly on most navigation software.

They are also raw. No forecaster has moderated them, they can resolve coastal effects poorly, and their grid spacing may be coarser than the feature you care about.

Use all three layers. They answer different questions.

#Reading model uncertainty honestly

The single most useful habit in offshore weather work is comparing models.

Also watch how successive runs of the same model evolve. A forecast that has been drifting in one direction over three runs is telling you something a single snapshot cannot.

Forecast skill degrades with time. Days one and two are usually actionable; day three or four is a planning guide; beyond that you are reading trends, not timings.

#Getting data at sea

The delivery method matters less than the redundancy.

Satellite broadband — Starlink and equivalents — makes full graphical weather, synoptic charts and multiple models trivially available. It also needs power, working terminal hardware, a subscription and network service.

Low-bandwidth satellite — messengers and older satellite data services — delivers text forecasts and compressed GRIBs on a fraction of the power, and keeps working when the vessel's main electrical system has a problem.

HF radio remains relevant to some long-distance cruisers for broadcast weather products.

Coastal VHF and mobile data cover most Australian coastal cruising perfectly well.

The resilient arrangement is a high-bandwidth path for convenience plus a genuinely independent low-power path for safety. That is the same layering logic as the satellite communications guide.

And budget the energy. A satellite broadband terminal running continuously can consume meaningfully more than the navigation sensors combined. On a sailing yacht where the autopilot already dominates consumption, that matters — see marine electrical power.

#Routing software and polars

Weather routing software combines model data with a performance model of your boat — its polars — and proposes a time-optimal or comfort-optimal track.

It is genuinely useful. It will find options a human would not think of, particularly on long passages where a detour to stay on the favourable side of a system pays off.

It is also only as good as three inputs:

1. The polars. Manufacturer polars are usually optimistic and assume a racing crew, clean bottom and full sail inventory. A cruising couple reefing early on a boat with a loaded waterline will not achieve them. Routing against optimistic polars produces routes that require boat speed you will not have.
2. The model. A stale GRIB, or one model taken as truth, produces a confident route built on sand
3. The constraints. The software does not know about your crew's seasickness, a tired autopilot drive, an engine you would rather not rely on, or an entrance you will not attempt at night.

Treat a routing solution as a proposal to critique. Ask what it assumes, what it would cost to be wrong, and what the fallback is.

#Weather-limited decisions worth pre-deciding

Before departure, decide the thresholds rather than improvising them:

• maximum wind and sea state you will accept for departure
• the conditions in which you will turn back or divert
• the wind directions that make each bail-out option tenable or untenable
• the point of no return on the passage
• whether you will attempt the destination entrance at night, or wait offshore for daylight

These belong in the passage plan in writing. A threshold agreed in calm conditions is far easier to honour than a judgement made when everyone is tired and wants it to be over.

#Australian specifics worth knowing

• East coast lows can develop quickly and produce conditions well beyond the preceding forecast confidence. Treat rapid deepening in the models seriously even when the current weather is benign.
• The Chart Datum in Australia 2026">East Australian Current can run strongly south along the New South Wales coast, and wind against current builds steep, dangerous sea out of proportion to the wind speed. This is a routing input, not a curiosity.
• Tropical systems in the northern summer change the planning problem entirely; cruising plans in the Radar Australia 2026">cyclone season are as much about where the boat will be as how it will sail.
• Sea breezes along much of the coast produce a reliable diurnal pattern that coarse global models handle poorly. Local knowledge and coastal waters forecasts beat GRIB here.
• Bass Strait and the Great Australian Bight combine fetch, shallow water and frontal passage; margins should be larger than the same wind speed elsewhere.

Confirm all forecast interpretation against current Bureau of Meteorology products rather than relying on generalisations.

#A practical routine

For an offshore passage:

1. Two to three days out: review synoptic prognosis and identify the pattern and the risks
2. One day out: compare two or three models, fix departure timing, confirm go / no-go against thresholds
3. Departure: download the latest GRIB set and official forecast; note what you were given so drift is visible
4. At sea: refetch on a schedule — commonly every 12 hours, more often near a decision point
5. Each fetch: compare against the previous, check warnings first, then pattern, then detail
6. Log the barometer. A falling trend that the model did not predict is the boat telling you the forecast is wrong

That last point is underrated. Your own observations — barometer, wind shift, cloud, swell direction and period — are real-time data the model does not have.

#Bottom line

Offshore weather work is not about acquiring the best data. It is about understanding how uncertain the data is, comparing sources, pre-deciding thresholds, and continuing to observe the actual sky and barometer once the model has told you what it thinks.