How to find a weather window for a coastal passage

Start with the hours you need

For a coastal trip lasting four hours to two days, start with your intended route and a realistic estimate of the time underway. Write down the departure, the main exposed sections and the arrival. Beside each one, put the time you expect to be there.

As a simple example, 36 nautical miles at an average speed over the ground of 6 knots takes six hours. At 5 knots, the same distance takes seven hours and twelve minutes. Those are arithmetic examples, not suggested speeds. Use an estimate that reflects your own boat and passage.

Then ask how much later you might arrive if progress is slower. Carry the weather check through that later time too. The RYA passage planning guide includes delays, tidal constraints and daylight among the details to consider. A window that ends at your earliest arrival leaves none of those timing questions answered.

Read the forecast for the water you will cross

Open the marine forecast for every area your passage crosses. Check when it was issued and the period it covers. A forecast published this morning may describe a different set of hours from the one you saved last night.

For UK coastal trips, the Met Office inshore forecast covers waters up to 12 miles offshore and includes wind, weather, visibility and sea state. Its marine forecast guide explains the areas and terminology. If your route extends beyond that coverage, choose the relevant marine forecast as well.

In the US, National Weather Service marine forecasts include coastal area forecasts and point forecasts. Read the area forecast and current warnings before narrowing the view to individual locations.

The broad forecast gives the trip a setting. The detailed forecast helps you follow what changes between the start, middle and end.

Follow the changes across the whole passage

Make a short note for each section of coast. Keep the same fields each time so you can see where the conditions change:

• Expected arrival time at that section.
• Wind direction, average speed and gusts.
• Wave height, period and direction.
• Visibility and any forecast showers, thunderstorms or fog.
• Whether conditions are building, easing or changing direction.

Pay particular attention to changes that happen before you finish. If the wind turns during the afternoon, look at where you expect to be at that hour. If waves are building toward evening, check the later sections rather than relying on the morning values.

Wind and waves deserve separate attention. The NWS explanation of wave detail identifies height, period and direction as the three basic properties of waves. A single height does not describe the whole sea state. Our guide to wave height and wave period explains what those numbers tell you.

A six hour passage with a late change

Imagine a 36 nautical mile trip that you expect to complete between 09:00 and 15:00. The forecast looks suitable for your boat and crew through the morning, but conditions on the final section begin to exceed your chosen limits at 16:00. These times are an invented example, not a forecast for a real place.

On the original estimate, you finish an hour before that change. But the slower estimate of seven hours and twelve minutes puts arrival at 16:12. The same weather window no longer covers the whole trip.

That is the useful question to ask: does the continuous period still cover the passage when the timing changes? You might investigate a shorter destination, another day or a different start. The point is to notice the mismatch while you are still planning.

For a passage lasting into a second day, repeat the check through the night and the following morning. Do not stop at the end of the first forecast screen.

How far ahead should you look?

Start looking several days ahead to identify possible periods. As the trip gets closer, focus on the timing of changes along your route. There is no single number of days that makes every forecast equally useful.

The Met Office explains that ensemble forecasts show a range of possible outcomes. Greater spread indicates more uncertainty. For a coastal passage, a useful application is to examine whether different outcomes move a weather change into the hours you expect to be underway.

Keep a note of the window you first identified and compare it with the next forecast. Has the change moved earlier? Is the suitable period shorter? That comparison is more useful than remembering that Saturday looked good two days ago.

Check the window again before leaving

Refresh the marine forecast and warnings, then look at recent observations where available. In US waters, the NOAA buoy observation map provides a starting point. Check the observation time and station location before comparing a reading with your forecast.

Return to your original notes: the hours needed, the exposed sections and the arrival conditions. If those still fit the latest forecast, you have a clear basis for comparing your options. For the next step, read how to compare departure times for a coastal passage.

Sources and further reading

• RYA passage planning
• Met Office guide to marine forecasts
• NWS marine weather services
• NWS coastal forecasts with wave detail
• Met Office ensemble forecast explanation
• NOAA buoy observations