Reading Marine Weather Forecasts: Advisories, Wind, Waves and Go/No-Go

Look up your zone the evening before to plan, then check again right before you leave. Forecasts are updated during the day, and a morning update can add an advisory that was not there the night before. Read the whole forecast period you will be out, not just "today".

Any active hazard appears at the top of the forecast in capital letters. NOAA explains the levels like this:

• Watch: a hazardous event is possible but not certain, usually in the next 12 to 36 hours. Make a backup plan.
• Advisory: hazardous conditions are coming or happening but are below warning level. For a small boat, this is often a no-go.
• Warning: dangerous conditions are likely, imminent or occurring and threaten life and property. Stay in port, or get there now.

A Small Craft Advisory (SCA) is issued for sustained winds or frequent gusts and/or seas that may be hazardous to small boats. The thresholds depend on where you are. The NWS definitions page lists them by region, for example:

• Maine to South Carolina, Lake Erie and Lake Ontario: about 25 to 33 knots (20 to 25 knots on the low end for harbors and bays in some areas) and/or seas of 5 to 7 feet or more, depending on the area.
• Georgia to Texas and the Caribbean: 20 to 33 knots and/or seas of 7 feet or more expected for over 2 hours.
• Washington to California: 21 to 33 knots and/or waves over 10 feet or locally steep waves.
• Great Lakes (Minnesota to Ohio): 22 to 33 knots and/or waves over 4 feet.

An SCA can also be issued for ice that is hazardous to small boats. Check the NWS page for your exact region's numbers.

These come from the NWS definitions, with winds in knots (mph in parentheses):

• Gale Warning: sustained winds or frequent gusts of 34 to 47 knots (39 to 54 mph).
• Storm Warning: 48 to 63 knots (55 to 73 mph).
• Hurricane Force Wind Warning: 64 knots (74 mph) or more, not tied to a tropical cyclone. Tropical storm and hurricane watches and warnings come from the hurricane centers.
• Special Marine Warning: a short-lived hazard, usually 2 hours or less, such as thunderstorm winds of 34 knots or more, large hail or waterspouts.
• Dense Fog Advisory: visibility down to a locally set limit of 1 nautical mile or less. See our boating-in-fog guide.

Wind direction is where the wind comes from (a north wind blows from north to south). Marine forecasts give speed in knots; one knot is about 1.15 mph. Pay attention to gusts. The NWS defines gusts as rapid swings of 10 knots or more between peaks and lulls, and a forecast of "10 to 15 knots, gusts to 25" is a very different day from a steady 15. Think about how the wind lines up with your route: a breeze in your face going out is in your back coming home, and the reverse can mean a wet, slow ride back.

The "seas" number is the significant wave height: the average of the highest one-third of the waves. NOAA notes that individual waves can be much bigger. With a 10-foot significant height, about 1 in 100 waves can top 16 feet. A range such as "seas 2 to 4 feet" shows uncertainty or changing conditions; it does not mean every wave falls in that range.

Where the forecast lists wave detail, look at the period (seconds between crests). Short-period waves are steep and choppy and the hardest on small boats. Long-period swell is gentler offshore but can stack up and break at inlets and shoals.

Waves build fast when the wind blows against the current, such as an outgoing tide in an inlet against an onshore wind. Environment Canada's marine tips add a few more points:

• Plan inlet crossings around the stage of the tide.
• Wind reported from a sheltered harbor can be much lighter than out on open water.
• If the forecast calls for heavier wind and waves outside your bay, believe it, even if the cove looks calm.

Read the forecast for thunderstorms, wind shifts and a cold front passing. Fronts often bring a sudden wind shift and gusts. The NWS says every thunderstorm produces lightning, and storms can bring shifting gusty winds, waterspouts and downpours. They can form fast; BoatUS notes a small cloud can grow into a storm front in about 30 minutes. If storms are in the forecast, plan to be off the water before they are expected, or stay in.

NOAA Weather Radio All Hazards broadcasts continuous NWS forecasts and warnings 24 hours a day on seven VHF frequencies (162.400 to 162.550 MHz). Most marine VHF radios have weather channels, often labeled WX, that receive it, and some radios and dedicated weather receivers can sound an alert when a warning is broadcast (check your radio's manual). Learn the place names and landmarks your local broadcast uses, as BoatUS suggests. When you have cell coverage, a radar app helps you see storms coming. Out of cell range, consider satellite weather.

Compare the forecast with what is actually happening. NOAA's National Data Buoy Center (ndbc.noaa.gov) maps weather buoys and coastal stations that report wind, gusts, waves, pressure and water temperature in near real time. BoatUS notes most update about once an hour. Pick one or two stations on your route and check them in the morning: if the buoy already shows more wind or bigger seas than forecast, believe the buoy. BoatUS also warns that some stations are seasonal or run on tight budgets, so have a backup station in mind.

Use a simple rule set and stick to it:

• No-go: any warning for your area, thunderstorms forecast during your trip, or an advisory when you run a small open boat.
• Think hard: winds or seas near the top of what you and your boat handle comfortably, a front due while you are out, wind against tide at your inlet, or a crew of first-timers.
• Go: conditions well inside your comfort zone for the whole trip, with a backup plan.

"When in doubt, don't go out" exists for a reason. Thunderstorms are usually brief, and the NWS points out that waiting one out ashore beats riding it out on the water.

Tell someone ashore where you are going, who is aboard and when you will be back. Write down the forecast you saw and your bail-out spots: the nearest harbors or coves you can duck into if the weather turns.

Check WX on the VHF every so often. Watch for darkening skies, a sudden drop or shift in the wind, building swell and distant lightning. BoatUS notes a low, dark, sharp-edged cloud front signals a stronger storm. Count the seconds between a lightning flash and the thunder and divide by five to get the distance in miles. If you hear thunder, you are close enough to be struck.