Choose the Japanese coast and passage type first: island weather, bay traffic and tidal straits need different evidence.
Ryukyu, Tokyo Bay and tidal straits are different plans
Use the Ryukyu island desk for a southern island passage; use Tokyo Bay observations and traffic information for a Yokohama departure. A Seto Inland Sea or strait passage needs its own local current information, not the bay forecast carried across the country. Start the brief with the intended coast and exposed legs rather than a general Japan city forecast.
Keep marine warning, wave chart and typhoon product separate
JMA offers distinct marine warnings/forecasts, wave charts and tropical-cyclone information. Select the named marine area that contains each exposed leg, then inspect the current issue and valid time of each product. A colourful track is not the complete hazard footprint, and a daily coastal forecast is not a tide or harbour-access instruction.
Do not use a seasonal description as a departure window
Seasonal trip planning and the next passage have different time scales. For an island itinerary, consider how a cyclone-related change would affect both the exposed leg and the intended refuge, and retain a route with a verified alternative. If a longer itinerary extends beyond forecast coverage, schedule new official checks; do not label the whole week suitable using the first day's bulletin.
Tide height, current and ocean circulation answer different questions
The Coast Guard tide/current index contains location-specific products. Use water level with its stated vertical reference for a depth question; use the relevant tidal-stream prediction for motion through a named strait. Do not infer slack water from high water at an unrelated station, or turn a broad ocean-current field into an entrance-current prediction. Record missing local information explicitly.
Match observation exposure and the actual port
For Tokyo Bay, follow the Coast Guard's local observation links and keep the station time, position and role in the brief. A sheltered marina reading does not characterise the bay entrance or an onward open-coast leg. Yokohama Bayside Marina, a commercial port approach and a generic Yokohama map pin are not interchangeable destinations; confirm the actual berth and responsible local authority.
A reproducible Japanese passage brief
Compare the original and alternative departure at the exposed-leg ETA and arrival ETA. For every number retain issuer, product, issue, valid time, units and time zone; JMA pages may use JST or UTC, so do not silently mix them. Add local tide/current and traffic checks, a slower-arrival scenario and the next warning update. The country page is a source guide; it does not authorise departure or expand the automated service's data coverage.
Official products by task
- JMA marine warnings and forecasts — Named Japanese marine areas; select the applicable area, current issue and valid period. This JavaScript product is not an automatic route-coverage match.
- JMA tropical cyclone information — Official tropical-cyclone analysis and forecast; read its issue, lead time and uncertainty separately from the ordinary marine forecast.
- JMA wave analysis chart — seas around Japan — An analysis chart for its stated time, not a forecast of the next passage. It shows significant wave height, not the maximum individual wave, and does not resolve a marina entrance or local tidal stream.
- JMA wave forecast chart — seas around Japan — A distinct forecast chart; preserve its issue and selected valid time for the exposed-leg ETA. Significant wave height is not an individual maximum, and a regional wave chart is not an entrance-current or breaking-wave assessment.
- Japan Coast Guard tide and tidal-current predictions — Separate tide heights from tidal streams; select the specific location or strait, date, time basis and datum. An index is not a prediction for every Japanese passage.
- Japan Coast Guard Tokyo Bay weather and ocean information — Local official links to observations, water levels, tides and current information; follow the underlying station or product and check its timestamp and exposure.
Waters and onward passages
- Japan: Ryukyu Islands
Island passages need national marine warnings and a separate tropical-cyclone check.
Departure ports and bases
- Yokohama: Yokohama Bayside Marina
A Tokyo Bay departure desk tied to a real visitor marina, local marine products and bay traffic information.
Map orientation
The approximate area centre is not an approach waypoint, a track, a depth survey or a forecast-zone boundary. The third-party map loads only after your click.
Prepare a passage brief
Bring route timings, vessel and crew profile together with checked sources and reassessment triggers. Personal AI analysis and expert review are in invitation-only closed testing. An editorial geographic page does not extend the calculation service’s actual coverage.