A marine forecast is three things stacked together: wind, seas, and whatever else is going on — fog, thunderstorms, rain. Most people read the wind and skip the rest. The rest is where the trouble usually is.
"Seas" is not the height of the waves you will meet
This is the single most misread line in any forecast, and it is misread in the same direction every time — optimistically.
Two things are being combined into one number. First, seas are the combination of both locally generated wind waves and distantly generated swell waves. Wind waves are short-period and built by the wind in front of you. Swell has travelled out of the area where it was generated and arrives smoother, more regular, with a longer period. Both are in that figure.
Second, the figure is a significant wave height — in the NWS glossary's words, "the mean or average height of the highest one third of all waves". It approximates what an experienced observer would report if estimating the sea by eye. It is not a maximum, and it is not close to one.
The National Weather Service puts numbers on the gap. For a forecast of 10 feet:
- 1 in 10 waves will be larger than 11 feet
- 1 in 100 waves will be larger than 16 feet
- 1 in 1,000 waves will be larger than 19 feet
And at an eight-second wave period, waves of 19 feet or more come round roughly every 2.2 hours. Not once in a freak season — a few times in a day out.
Apply the same ratios downward and the point still holds at sizes people actually fish in. A 4-foot forecast puts about 1 wave in 100 over 6 feet. That wave is not dangerous on its own. It is dangerous because it arrives while you are side-on, netting a fish, or standing up.
The advisory ladder
Advisories key off sustained wind:
| product | sustained wind | |---|---| | Small Craft Advisory | 20 to 33 knots | | Gale Warning | 34 to 47 knots | | Storm Warning | 48 knots or more |
A Special Marine Warning is different in kind, not degree. It is issued for a hazard of short duration — "usually of short duration (2 hours or less)" — producing winds of 34 knots or more that the standing forecast does not adequately cover. In practice that means a thunderstorm or a waterspout. If one is issued while you are out, it is about the next hour, not the afternoon, and it is the one to act on immediately.
"Small craft" does not mean what most people think
There is no legal definition for "small craft". The advisory is not a category your boat is either inside or outside of.
The question it actually asks is whether your boat, loaded the way it is today, with the people you have aboard, would be adversely affected by those conditions. A 24-foot centre console with two experienced adults and the same boat with children aboard are not the same vessel in a 25-knot wind. The advisory hands you the conditions; you supply the judgement.
The thresholds are local — don't memorise a number
Criteria differ between forecast offices, because the water differs. Two real examples:
- Key West issues a Small Craft Advisory for winds of 20 to 33 knots, and uses seas of 7 feet or greater.
- New York issues one for sustained winds or frequent gusts of 25 to 33 knots, and/or seas of 5 feet or higher.
Same product name, different trigger, and a 6-foot forecast means something different in each. Find your local forecast office's criteria once, and read its forecasts against those — not against a number you picked up somewhere else.
A forecast has a shelf life
The NWS marine forecast FAQ is blunt about this: "Most marine forecasts are updated every six hours and as conditions warrant."
A forecast you read the night before is at least one revision out of date by the time you back down the ramp, and the revision you missed is the one that included the overnight observations. Read it again in the morning. If you only do one thing differently after this guide, do that one.
Reading one in practice
Before you go, take four things in this order:
- Any advisory or warning in force — and when it starts and ends, not just whether one exists.
- Sustained wind and gusts for the hours you will actually be out, rather than the daily summary. A day that averages fine can have an ugly afternoon.
- Seas, read as the average of the biggest third — so ask yourself what the 1-in-100 wave looks like, and whether your boat and your crew are fine with it.
- Visibility. Fog changes which rules you are operating under. Restricted visibility has its own navigation rules and sound signals, and they are worth knowing before you need them — our course covers them in Restricted Visibility (Rules 19, 35).
If that is more assembly than you want to do at five in the morning, the trip planner reads the same NOAA data for your launch point and turns it into a go or no-go call, showing which factors it could and could not weigh. The interactive map is the visual version — radar, wind and severe-weather warnings drawn over the water you are heading for.
Before you go
Whatever the forecast says, tell someone ashore where you are going and when to worry. That is a float plan, it takes two minutes, and it is the thing that starts a search if the forecast turns out to have been the optimistic version.