In short
- A watch means the conditions are possible; a warning means they are expected or happening; an advisory is the lower tier, for conditions that matter to smaller boats.
- In US waters the ladder runs: small craft advisory up to 33 knots, gale warning 34 to 47, storm warning 48 to 63, hurricane force wind warning 64 and over.
- A special marine warning is short-fused: thunderstorms with 34 knot gusts, large hail or waterspouts, usually for up to about two hours.
- Read the start and end times. A warning that begins at 14:00 says nothing about a passage that ends at noon, and everything about one that ends at 16:00.
Watch, warning or advisory
The National Weather Service uses three tiers across all its products, and the marine ones follow the same logic.
- Watch: the risk of a hazardous event has risen, but its timing or location is still uncertain. A gale watch is typically issued a day or two ahead.
- Warning: the event is happening, or is expected with enough confidence to name a start and end time.
- Advisory: conditions that are less severe than warning level, but significant for the people the advisory is aimed at. For marine products that usually means smaller boats.
Every one of them carries an issue time, a start time and an expiry time. Forecasters extend, cancel or upgrade them as the weather develops, so the version you read last night is not necessarily the one in force this morning.
The wind ladder: advisory to hurricane force
Most marine wind warnings are defined by sustained wind or frequent gusts in knots. The bands line up neatly with the Beaufort scale: a gale warning starts exactly where force 8 starts.
- 1Small craft advisory. Its lower edge is set regionally, between about 20 and 25 knots; its upper edge is 33 knots.
- 2Gale warning: 34 to 47 knots, Beaufort force 8 and 9.
- 3Storm warning: 48 to 63 knots, force 10 and 11.
- 4Hurricane force wind warning: 64 knots and over, force 12, outside a tropical cyclone.
- 5The Beaufort forces for the same wind speeds.
When the wind comes from a tropical cyclone, the names change but the speeds stay similar. A tropical storm warning means sustained winds of 34 to 63 knots are expected within 36 hours, and a hurricane warning means 64 knots or more within 36 hours. The matching watches go out 48 hours ahead.
| Product | Main trigger | How far ahead |
|---|---|---|
| Small craft advisory | Sustained wind or frequent gusts from about 20 to 25 knots up to 33, or seas above a regional height | Usually issued within a day or so of onset |
| Gale warning | Sustained wind or frequent gusts of 34 to 47 knots | Hours to a day or more |
| Storm warning | 48 to 63 knots | Hours to a day or more |
| Hurricane force wind warning | 64 knots or more, not from a tropical cyclone | Hours to a day or more |
| Hazardous seas warning | Seas high enough to be hazardous below gale force | Hours to a day or more |
| Special marine warning | Thunderstorm gusts of 34 knots or more, hail of 3/4 inch or more, or waterspouts | Minutes, valid for up to about 2 hours |
| Dense fog advisory | Visibility of 1 nautical mile or less, widespread | Hours |
| Freezing spray advisory or warning | Spray freezing on to the boat; a warning for heavy accumulation | Hours |
The small craft advisory
The small craft advisory is the one coastal boaters see most, and the one that causes the most confusion. There is no legal definition of a small craft. The advisory says conditions are hazardous to smaller vessels, and it leaves you to judge how that applies to your boat and crew.
Its trigger is set by each NWS region, so the same wind can bring an advisory on one coast and not another. The upper limit is always 33 knots, because 34 is a gale. The lower limit sits somewhere between about 20 and 25 knots, and many offices also issue it for seas above a local height, often somewhere between 4 and 7 feet, even when the wind is lighter. Variants make the cause explicit: a small craft advisory for hazardous seas when the sea is the problem rather than the wind, and on the Pacific coast a small craft advisory for rough bar when breakers are expected on a harbour bar.
The useful habit is to read past the headline to the forecast underneath. "Small craft advisory: SW 20 to 25 kt, seas 4 ft at 5 seconds" and "small craft advisory: NE 15 kt, seas 8 ft at 12 seconds" are very different days on the water.
Special marine warnings and other short-fused products
A special marine warning covers a specific, fast-moving hazard, nearly always a thunderstorm or line of storms. It is issued when a cell is producing or expected to produce gusts of 34 knots or more, hail of 3/4 inch or larger, or waterspouts. It names the area, the direction the storm is moving and the time it reaches particular places, and it usually runs for up to about two hours.
Below that level, forecasters issue a marine weather statement for storms with gusty winds that fall short of warning criteria. Neither shows up days ahead. On a summer afternoon with thunderstorms in the forecast, the useful planning step is to look at the squall risk for the hours you'll be out, and to keep a way to receive warnings while you are underway.
A dense fog advisory goes out when widespread fog brings visibility down to 1 nautical mile or less. Fog can hold all day on a coast with a cold sea and light wind, so its end time is often vague. Treat it as a strong hint about the hours around dawn and the first leg of a morning departure.
Gale and strong wind warnings in the UK
The Met Office issues gale warnings for each shipping forecast area when winds of force 8 or more are expected, or gusts high enough to match. Each warning uses the timing words imminent (within 6 hours), soon (6 to 12 hours) and later (more than 12 hours), counted from the time of issue. Severe gale, storm and violent storm follow the Beaufort forces up the scale.
Closer in, the inshore waters forecast adds strong wind warnings for force 6 or 7, which is the level where many cruising boats reef hard or stay in. They are broadcast with the forecast and on VHF by HM Coastguard, so you can hear them underway.
Planning a departure around a warning
- Read the times, not just the headline. Note when the warning starts and ends, and compare it with your departure and your arrival at each waypoint.
- Read the forecast underneath. Wind direction, sea height and period decide how the conditions feel on your course. A beam sea and a following sea at the same height are not the same trip.
- Check where it applies. Warnings are issued by zone. A warning for the outer waters may not reach the sheltered sound you're using, and the reverse is true too.
- Look at the trend. A gale watch for tomorrow that becomes a warning tonight is a forecast getting more confident. One that is cancelled is the opposite.
- Find the window either side. Most warnings are shorter than they first look. The hours before the start and after the end are often a good window, especially once the sea has had time to go down.
An afternoon small craft advisory
The coastal waters forecast for your zone carries a small craft advisory in effect from 13:00 to 22:00: "SW winds 20 to 25 kt. Seas 4 to 6 ft." The morning period reads "SW 10 to 15 kt. Seas 2 to 3 ft."
Your passage is 26 nautical miles at 6 knots, four hours and twenty minutes. Leave at 07:00 and you arrive at 11:20, almost two hours before the advisory starts, in the lighter morning breeze. Leave at 10:00 and the last hour falls inside it, on the most exposed leg. The decision is not about the advisory as a label; it is about which hours of your trip it covers.
If the advisory is for hazardous seas behind a passing front, the sea often takes longer to settle than the wind. Give it a few hours after the end time, and look at the period: a long, decaying swell is far kinder than the short sea that was there the evening before.
