A Master is responsible for every passage planning decision, and weather is the single variable most likely to invalidate a plan that was sound at the time of departure. Understanding where your information comes from, how accurate it is, and when it is no longer reliable is a command responsibility — not a watchkeeper skill.
Why multiple sources matter
No single forecast is authoritative. Numerical weather prediction (NWP) models diverge beyond 72–96 hours; mesoscale effects (sea breezes, orographic acceleration, thermal lows) are poorly resolved in global models. A Master must triangulate between synoptic forecasts, local observations, and direct instrument reading to build a coherent picture and identify where the uncertainty lies.
Sources available to the Master
- Synoptic charts and text forecasts: NAVTEX (up to ~400 nm), SafetyNET (global, via Inmarsat/VSAT), Météo-France, Met Office, national HF radio broadcasts. Each carries an implied age — NAVTEX weather messages repeat at scheduled intervals and may be hours old by the time they are read.
- Coastal and port-specific forecasts: VHF met broadcasts from coast stations; harbour authority or pilot information.
- Commercial routing services: gribs delivered via satellite or SSB/Winlink; give high spatial resolution but require the Master to understand the underlying model (GFS, ECMWF) and its known biases.
- On-board instruments: aneroid barometer or electronic pressure sensor, barograph, thermometer, hygrometer, anemometer/wind vane. These give actual conditions and are the primary cross-check against forecast.
- Direct observation: sea state, swell direction, sky appearance, cloud type sequence. Classic synoptic indicators (backing/veering wind, falling pressure trend, cirrus followed by altostratus) remain entirely valid operational tools.
Pressure tendency and the barograph
Rate of pressure change is more operationally significant than absolute value. A fall of 1 hPa per hour sustained over three hours signals a rapidly deepening system. The barograph provides the trace — the Master should note the tendency at watch handover and record it in the deck log.
Weather systems at command level
The Master must be able to identify a deepening Atlantic low, a Mediterranean genoa low (rapid intensification, short fetch but steep sea), a mistral/tramontane regime, and a tropical revolving storm (TRS) — including the navigational action rules (Buys Ballot, dangerous/navigable semicircle, manoeuvring strategy). Failure to demonstrate TRS avoidance is a common reason for referral at oral.
Reporting and recording
Voluntary Observing Ships (VOS) schemes allow Masters to submit synoptic weather observations to national meteorological services; participation is encouraged but not mandatory for yachts. What is mandatory is that weather observations and significant forecasts received are entered in the deck log, that barometric readings are recorded, and that the passage plan file retains the forecast products used for planning. If conditions deteriorate and the plan is amended, the Master must record the reason, the forecast received, and the decision made. This is evidence of proper command judgement, not paperwork for its own sake.
The command decision
The question an examiner will push you toward is: at what point does the forecast, cross-checked against your instruments and observations, require you to delay departure, alter route, or seek shelter? That decision must be documented and defensible.