Module 13 - Meteorology
Advanced Scenario Check: Forecast Mismatch and Tidal Gate
What you will learn
Combine an earlier-than-forecast wind increase, a quick pressure fall, reduced speed, crew condition and a tidal entrance gate into one timed diversion decision.
A weather plan fails when it names the front but ignores the shrinking operating margin. The defensible answer recalculates the route, chooses the easy fallback and gives the crew a clear review point.
At 1200 in this fictional exercise, the yacht is 18.0 nautical miles from an exposed entrance that must be reached by 1545. The forecast increase to Force 6 has arrived early, pressure has fallen 5.3 hPa in three hours, the sea is building against the stream and monitored speed over the ground has reduced to 4.5 knots. One crew member is seasick and the easier harbour is 7.5 nautical miles away on a route expected to make 5.0 knots.
Separate the evidence. The pressure tendency is falling quickly; the early Force 6 and building sea contradict the expected timing; the reduced speed is measured progress; the crew limitation is operational. None alone dictates the destination, but together they remove the original timing and workload margin.
Test the exposed route before debating the weather-system label. Eighteen miles at 4.5 knots takes four hours, giving a 1600 arrival — fifteen minutes after the fictional 1545 gate and before adding any manoeuvring or contingency allowance. The original route therefore fails on the monitored case.
The easier harbour is 7.5 miles away. At 5.0 knots the estimate is one hour thirty minutes, giving a 1330 arrival from 1200. That figure is not a guarantee; it shows why the fallback remains substantially easier now than after another hour towards the exposed entrance.
Brief the diversion, sail and watch plan, casualty care, weather-update task and a review point. Reopen the original route only if a fresh information set, observed conditions, crew state and a recalculated arrival all restore the chosen margin. This is written revision, not a live forecast or route recommendation.
Worked example
Compare the exposed entrance with the easier harbour using the stated fictional distances and monitored speeds at 1200.
- 1Exposed route time: 18.0 M ÷ 4.5 kn = 4.0 hours.
- 2Exposed arrival: 1200 + 4 hours = 1600.
- 3Gate result: 1600 is 15 minutes after the 1545 latest time, before any extra contingency. The route fails.
- 4Fallback time: 7.5 M ÷ 5.0 kn = 1.5 hours.
- 5Fallback arrival: 1200 + 1 hour 30 minutes = 1330. Divert while that option remains straightforward, then continue monitoring.
Sense check: The original forecast may still be valid as a regional product, but its timing no longer describes the yacht's observed sequence and the monitored route calculation misses the gate.
| Evidence | What it establishes | What it does not prove |
|---|---|---|
| 5.3 hPa fall in 3 h | Met Office ‘falling quickly’ tendency band | The identity or exact arrival of a front |
| Force 6 by 1200 | Forecast timing and observation no longer align | One certain future wind sequence |
| 18.0 M at 4.5 kn | 1600 monitored arrival misses the 1545 gate | That speed will remain constant |
| 7.5 M at 5.0 kn | 1330 fallback estimate preserves an easier option | Guaranteed shelter or entry |
Give a weather diversion brief the crew can execute
The crew know the destination but have not yet been told that the exposed entrance is closed by the revised timing.
1. Facts
State the 1200 time, quick pressure fall, early Force 6, building sea, measured speeds, crew condition and both route calculations.
2. Decision
Close the exposed entrance plan and name the easier harbour, course, sail plan and immediate helm action.
3. Roles
Assign helm, lookout, navigation and weather update while giving one person responsibility for the seasick crew member.
4. Review
Set the next timed position and weather review and state that the original route reopens only when evidence, crew and arrival margin all pass.
Sense check: A useful brief lets every crew member repeat the destination, immediate action, their role and the next review point.
Common mistake or limitation
- Continuing because the regional forecast is still valid even though the observed timing and route margin have changed.
- Using the barometer to name a front with certainty instead of combining independent forecast and observation evidence.
- Calculating only the original destination and never comparing the time and workload of the fallback.
- Announcing a diversion without assigning navigation, lookout, weather and casualty roles or a review point.
Recap
- A forecast mismatch is operational evidence even when the weather-system diagnosis remains uncertain.
- Recalculate the route with monitored progress and test the gate before defending the destination.
- Compare the fallback while it is still easy, not after the original route has consumed the margin.
- Brief action, roles, review point and the exact conditions required to reopen the plan.
Optional quick check
Section 15 of 15
At 1200, 18.0 M at 4.5 kn gives 1600 for a 1545 gate; the easier harbour is 7.5 M away at an estimated 5.0 kn. What is the defensible decision?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory course
- MCA MGN 599 Amendment 1 — weather updates and contingency planning
- Met Office guide to marine forecasts
- Met Office marine forecasts glossary
- Met Office guide to synoptic weather charts
- Met Office guide to weather fronts
Continue studying Meteorology
This public lesson is part of Revision Module 13. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.