Tropical Revolving Storm Decision
A 25,000 GT general cargo vessel is on passage in the western North Pacific in August. The vessel is making 14 knots. The master receives a warning of a tropical revolving storm centred about 300 NM to the northwest, moving west-northwest at 12 knots. The vessel's intended track passes about 120 NM to the south of the forecast centre. The vessel has a maximum speed of 14 knots and is loaded with a deck cargo of timber. The barometer is falling slowly, and the wind is from the northeast, backing. Visibility is good. The next port is 4 days away, and there is no convenient alternative port. The master must decide whether to maintain course, alter to the south, or alter to the north.
1. Using the information given, determine the vessel's position relative to the storm and the safest course of action, explaining your reasoning.
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In the northern hemisphere, a TRS has counter-clockwise winds. With the centre to the northwest and the wind from the northeast backing, the vessel is in the navigable semicircle (the left-hand semicircle in the northern hemisphere) if the wind is veering, but backing wind suggests the vessel is ahead of the storm track or in the dangerous semicircle depending on the exact bearing. The key facts: centre to the northwest, storm moving west-northwest, vessel to the south of the track.
If the vessel is south of a west-northwest-moving storm, it is in the left-hand (navigable) semicircle. The safest action is to alter course to the south, putting the storm further astern and increasing distance, while maintaining speed. Altering north would take the vessel toward the storm track and the dangerous semicircle.
The judgement is to put the storm on the vessel's quarter, increase distance, and monitor the barometer and wind shift. The vessel should not attempt to pass ahead of the storm, because the storm's forward speed plus the vessel's speed may not give enough margin.
2. Explain the meteorological signs you would monitor to confirm the decision, and the limitations of the forecast.
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Signs to monitor:
- Barometer: a steady fall indicates approach; the rate of fall and the diurnal variation pattern matter. A fall exceeding about 1 hPa per hour is significant.
- Wind direction: in the northern hemisphere, a veering wind in the dangerous semicircle and a backing wind in the navigable semicircle. The shift confirms which side the vessel is on.
- Swell: a long, low swell from the storm direction often precedes the storm by many hours.
- Cloud and sky: cirrus, then altostratus, then cumulonimbus and squalls.
Limitations: the forecast track has an uncertainty cone that widens with time; the storm may recurve or intensify; the vessel's own speed estimate and the storm's forward speed are both uncertain. The master should use the latest warnings, plot the storm's position and forecast track on the chart, and set a minimum distance criterion. The decision should be reviewed at least every few hours, and the vessel should be prepared for heavy weather: secure the timber deck cargo, check lashings, and reduce speed if necessary.