Understanding marine weather forecasts

23rd March 2021 Jack O'Rourke

A marine forecast is a description covering the conditions expected over the next 24 hours over open water. The environment is ever-changing so keeping an eye out for storms, waves, winds, and extreme weather helps keep the fun safe. 

Let’s look at how to read a marine forecast in six easy steps:

1) Check the wind strength and direction

First, look at the wind speed. Depending on your experience and confidence, you need to choose a speed that’s suitable for your adventure.

Novice sailors will usually want to stick with conditions below force 4: up to 16 knots of wind. Less experienced boaters typically go out in up to 20-25 knots of wind, or up to a force 5. Sailors who have many nautical miles under their belt may choose to venture out to sea even in a force 6, 7 or 8: 22 to 40 knots of wind. Many of the saltiest sailors will prefer to stay in port in these conditions, because they know how dangerous the sea can be.

Something very important to remember is that you need to check the wind gusts speed, too. This is typically 5 to 20 knots greater than the sustained wind speed, and can be exaggerated around headlands or high coastlines. So you need to be ready to deal with gusts blowing at a higher speed.

In terms of wind direction, you can either read a basic report, which will tell you where the wind is coming from. Or you could look at a map on an app and see a simulation of the wind blowing.

2) Look at the weather radar

Check the weather: will it be sunny or rainy? Is there a chance of thunderstorms? It’s prudent not to go out to sea if there is a risk of long rain showers, as they reduce visibility as well as crew morale. And if the forecast says there may be a thunderstorm, it’s always best to stay put.

3) Look at the swell and wave height

Typically, the wave height will be aligned with the wind speed. So if the wind isn’t blowing strong, there will be smaller waves.

However, the swell comes from much further away. This can sometimes be a problem, as swell and wind-waves can travel in different directions. Check the swell direction, height and period. Beware: the swell itself can come from two directions at the same time. This is known as a cross swell.

Typically, a beginner boater will want to go out in waves and swell up to 1, 1.5 meters (3.3ft to 4.3ft). A confident sailor will leave port with a forecast of up to 3m (10ft), while a more experienced boater will be able to face swell and waves up to 5m (16ft). But bear in mind that if you need to take your boat through a shallow marina entrance or cut, even a small swell can be dangerous.

4) Currents

Next, check if any currents affect the area you are planning on sailing. Different boats will be more or less affected by currents. Engineless craft and small to medium sailboats avoid travelling against a current that flows at 1-2 knots or more. Bigger sailboats, racing boats and powerful motorboats can counteract currents moving at up to 5 knots. However, their speed and consumption will be less good.

5) Tides

Then, it’s time to look at the tide table. Find out what time high and low tide are, and figure out if they affect you in any way.

For example, if the tide is opposed to a current, the ocean will generate some confused, sharp waves and the ride will feel less comfortable. Check if the tide affects your departure or arrival - will you have to sail against the tide to enter or exit an anchorage? Will there be enough water where you plan to anchor?

6) Fog and visibility

Finally, look out for any fog or reduced visibility. You’ll want to avoid going out to sea in anything more than a haze.

Beware: fog is not always accurately forecast. For this reason, it’s best to leave port with the right gear for reduced visibility. This includes an air horn or marine whistle, a flashlight and a VHF radio. On bigger craft, an AIS is recommended.