Understanding weather forecasts for coastal sailors

Start with the inshore waters forecast, not the app on your phone

Before you look at anything else, read the inshore waters forecast for your stretch of coast. It covers the area up to about twelve nautical miles offshore and is written specifically for small craft, which means it speaks your language: wind direction and speed in Beaufort and knots, sea state, weather, and visibility. It is issued several times a day, and the wording changes subtly between editions, so it pays to read two or three in a row. A forecast that drifts from "southwest 4 or 5" to "southwest 5 to 7" over a morning tells you the situation is developing faster than the models first suggested.

Pair it with the shipping forecast for the wider picture and a coastal station forecast for your immediate neighbourhood. The three together give you context, trend, and detail. If they disagree, treat the more cautious one as your working assumption.

Wind speed, gusts, and why the "G" matters more than the average

Wind speed is the headline number, but the gust figure is what actually decides your day. A forecast of force 4 gusting force 6 sounds comfortable on the first half and lively on the second. In practical terms, if you sail a small keelboat or a dayboat, sustained force 4 is a lovely afternoon; occasional force 6 gusts in the same window will have you reefed early and watching the gusts roll down off the hills.

• Force 3–4 (7–16 knots): comfortable for most crews, good for learning and for light displacement boats.
• Force 5 (17–21 knots): rewarding sailing if you reef early and the sea has not had time to build.
• Force 6 (22–27 knots): serious for a small boat. Shorten sail before you leave the harbour, not after.
• Gusts two forces above the mean: expect them to arrive with showers, squall lines, or a sea breeze front. Reef for the gust, not the average.

Watch for words like "squally", "thundery", or "showers" attached to a wind forecast. Those flags mean the mean wind speed is almost meaningless for short periods, and a sudden thirty-knot gust can arrive with little warning. If the forecast mentions a squall line or a frontal passage in your window, plan to be in harbour or close to it.

Visibility, fog, and the charts that keep you out of trouble

Visibility is the most under-read part of any forecast, and the one most likely to turn a pleasant sail into a tense crawl. It is usually expressed in metres or nautical miles, with a rough ladder of good, moderate, poor, and fog.

• Good (over 5 nautical miles): normal coastal pilotage, no special measures needed.
• Moderate (2–5 nautical miles): keep a proper lookout, use the radar reflector, and know your escape harbours.
• Poor (1000 metres–2 nautical miles): a handheld GPS and a working VHF stop being luxuries.
• Fog (under 1000 metres): do not leave the berth for a pleasure sail. Even with electronics, small craft are hard to see on radar.

Sea fog is the classic coastal trap. A warm, moist airstream over a cold sea can produce fog patches that thicken and clear within an hour, often inshore of a headland and clear just a mile offshore. If the forecast says "fog patches" or "mist patches", assume your particular bay may be one of them.

Tides, sea state, and the wind-against-tide problem

A force 5 with the tide is a brisk but manageable sea; the same force 5 against a spring tide in a shallow estuary or over a sandbank can produce steep, breaking water that stops a small boat dead. Read the sea state line carefully — slight, moderate, rough, very rough — and then check your tidal stream atlas to see when wind and water will be fighting.

Wind over tide is worst at the turn of the tide and in the hour either side. If your plan has you beating home through a narrow channel at exactly that moment, either leave earlier or accept a longer day. Swell is also worth checking: a long, low swell is usually harmless, but a short, steep swell combined with wind over tide can make an entrance genuinely dangerous.

The sea breeze: an afternoon habit worth planning around

On a warm, settled day, the sea breeze will fill in during the late morning and build through the afternoon. It usually arrives from the seaward direction, adds two or three forces to the ambient wind, and dies away in the evening. That is often perfect sailing, but it also means the forecast for 09:00 tells you very little about 14:00.

Look at the expected maximum rather than the morning value, and plan your return leg for the late afternoon when the breeze eases. A day that starts with glassy water and force 1 can easily finish with a reef in and spray over the bow.

Making the call: day sail or harbour stay

Weigh the forecast against the crew and the boat, not against a generic standard. A well-found yacht with an experienced crew handles force 6 comfortably; a small open dayboat with children aboard does not. Ask yourself three questions: can the boat take it, can the crew take it, and can I get back if it turns out worse than forecast?

If the answer to the last one is uncertain, stay put. A morning spent checking warps, reading the harbour's maritime heritage boards, or simply watching the weather from the cockpit is not a wasted day. The sea will still be there tomorrow, and the forecast will have moved on. Knowing when to go is a skill; knowing when not to is seamanship.