Guide · Passage · planning

How should you plan the weather for a passage?

Leg by leg, against limits you declare in advance. A point forecast for departure says little about hour 14 of a Channel crossing. Passage splits the route into legs, works out when you will occupy each one under slow, nominal and fast scenarios, and evaluates wind, gusts and sea state across that whole window — then checks every leg against your declared limits, so the go/no-go call stays yours and stays explicit.

Worked example · Cherbourg → Plymouth demo briefing

Forecast wind by leg across each leg's ETA window · departure Sun 12 Jul, 06:00 UTC

View as table

Leg	Distance	Sustained	Gusts to	Seas to
L1 · Passe de l'Ouest → Omonville	7.91 nm	14–15 kt	25 kt	0.8 m
L2 · Omonville → Cap de la Hague	9.26 nm	19–20 kt	25 kt	1.3 m
L3 · Hague → Casquets TSS	32.6 nm	20–25 kt	32 kt	1.9 m
L4 · Casquets → Mid-Channel	24.83 nm	19–27 kt	35 kt	2.3 m
L5 · Mid-Channel → Eddystone	24.77 nm	17–20 kt	34 kt	2.0 m
L6 · Eddystone → Plymouth	6.92 nm	12–15 kt	27 kt	1.5 m

Six legs, ~106 nm. Each band spans the sustained wind forecast across the hours the boat could occupy that leg; the tick marks the strongest forecast gust in the window. This is Passage's built-in demo briefing — a fixed example scenario with a 51-member ensemble, not a live forecast.Source: Passage demo briefing snapshot · route cherbourg-plymouth-v1 · profile coastal-cruising-cautious.

Why legs, not points

Uncertainty about your own boat speed turns a route into a set of time windows. In the example, the Casquets leg could be occupied anywhere from 08:38 to 17:03 UTC — nearly nine hours — depending on whether the passage runs slow or fast. Conditions must hold across the whole window, not just at the optimistic ETA. That is why the bands in the chart are ranges: they are the forecast over every hour you might actually be there.

Declared limits make the verdict honest

The demo profile declares a 28-knot gust limit before looking at any weather. The briefing's verdict is then mechanical, not persuasive: all 51 forecast scenarios exceed that limit on the Casquets leg around 16:00 UTC, and the models also disagree enough mid-Channel that the honest answer is reassess after the next model run — which the briefing dates. See how forecast risk is measured along a route for the full risk arithmetic.

Methodology

Passage evaluates forecast conditions hourly across each leg's ETA occupancy window under slow, nominal and fast boat-speed scenarios, course-relative, with tides and currents applied. Ensemble members are checked individually against the declared limits, every claim in a briefing links back to its evidence, and completed forecasts are later verified against the weather that followed.

Limitations

The worked example above is a fixed demo scenario, not a live forecast. Wave figures come from a deterministic wave model — no wave ensembles exist. Ensemble exceedance counts are raw scenario fractions, not calibrated probabilities. Passage is a planning aid: it does not replace a current forecast, official marine warnings, or the skipper's judgment.

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