How to read a surf forecast: swell, period, wind and tide

What a surf forecast tells you

Almost every surf forecast, whether it comes from the National Weather Service or a surf website, gives you the same handful of numbers, in more or less detail:

• Height of the waves, in feet.
• Period, in seconds: the time between one wave crest and the next.
• Direction: where the swell comes from. “SSW” means it’s arriving from the south-southwest.
• Wind: how strong it is and where it blows from.
• And, separately, the tide.

None of these means much on its own. A 4-foot forecast can be a perfect day or a blown-out mess. What matters is how they add up at your beach. Let’s take them one at a time, then read a whole report.

Wave height: what significant wave height means

No two ocean waves are the same, so the number in a forecast isn’t the height of every wave. It’s the significant wave height: the average of the highest third of the waves. NOAA notes that it lines up well with what a skilled observer would estimate by eye.

The practical upshot is that there will always be waves bigger than the number in the forecast. NOAA gives these rough relationships:

Wave	Compared with significant height	With a 4 ft forecast
Most common	about half	2 ft
Average	0.64	2.6 ft
Average of the highest 10%	1.27	5 ft
Highest to expect	about 2	8 ft

A few times a day, one comes through at almost double. Scripps’ Coastal Data Information Program (CDIP), which runs wave buoys along U.S. coasts, puts it simply: the highest wave in a buoy record is likely to be about twice the reported height. Some NWS offices now also give the average of the highest 10% as “waves occasionally to” a given height.

There’s another catch: the forecast height is worked out offshore. Wave models such as NOAA’s WAVEWATCH III, Météo-France’s and the European Centre’s (ECMWF) run on a grid of points in open water, miles from the beach. As the waves reach the coast, the bottom slows them, steepens them and makes them break. How much they grow depends mostly on the period and the shape of the bottom. That’s why some forecasts add a separate “surf height” or “breaking wave height,” which is an estimate.

Swell period: how much power is coming

What period tells you

Period is the number people skip, and the one that changes the waves the most. It’s the time, in seconds, between two crests passing the same point. The longer it is:

• the farther away the waves formed, and the cleaner they arrive;
• the more energy they carry at the same height;
• the sooner they feel the bottom, and the more they grow when they break.

A simple rule of thumb:

• Under 6 seconds: local wind swell. Short, choppy, shapeless waves.
• 7 to 9 seconds: nearby swell or a well-developed wind swell. It can work if the wind cooperates.
• 10 seconds or more: true groundswell, with clear sets. NOAA says waves with periods longer than 10 or 12 seconds are arriving from a distant source and count as swell.

In practice, a 3-foot swell at 8 seconds and a 3-foot swell at 14 seconds are very different days. TidePlanet estimates the surf at the shore with the Komar and Gaughan breaking-wave formula: on an open beach, the first gives sets of just under 4 feet, and the second, about 4.5 feet. We explain why in our guide to swell period.

Groundswell vs. wind swell

A good forecast splits the ocean into two parts:

• Wind swell (also called wind waves or sea): waves raised by the wind blowing in your area. They’re short, a few seconds apart, with whitecaps.
• Groundswell: waves that formed far away, in a storm, and traveled hundreds or thousands of miles to your coast. They arrive in clean lines with long periods.

Together they make up the combined sea, which is what buoys measure and what the total height in a forecast refers to. For surfing, you want a clean groundswell with little wind swell on top. If the wind swell carries a big share of the energy, the waves arrive jumbled even when the total height looks good. And when two swells from different directions overlap, they cross and form peaks that shift around.

Swell direction and the way your beach faces

Swell direction is where the swell comes from, just like wind direction. A northwest swell hits a beach that faces northwest head-on, a beach that faces west at an angle, and barely reaches a beach that faces southeast.

To read it:

1. Work out which way your beach faces: the direction of open water as you look out from the sand.
2. Compare it with the swell direction.
3. The bigger the angle between the two, the less swell gets in.

In TidePlanet’s surf score, the full swell reaches the beach up to about 35° off; at 70°, 80% of it; at 100°, half; at 135°, a quarter; and from behind, about 15%, the part that wraps around points and headlands.

This is how you pick a beach. When a huge swell hits an exposed stretch of coast head-on, the beaches facing it can close out, while spots that face away or sit inside a bay get less and may be rideable. With a small swell, do the opposite and look for the most exposed beach.

Wind: offshore or onshore

Day to day, wind changes the waves more than anything else. What matters isn’t just how strong it is, but where it blows from relative to the beach:

• Offshore wind blows from the land out to sea. It holds up the face of the wave, makes it peel and leaves it glassy. It’s the best, as long as it’s light: strong offshore wind lifts the lip and makes it hard to get into waves.
• Onshore wind blows from the ocean onto the beach. It pushes the wave from behind, makes it break too early and fills it with chop.
• Cross-shore wind blows along the beach: some chop and a drift down the beach.
• No wind: glassy water.

To find the offshore direction for your beach, think about the way it faces. Huntington Beach faces south-southwest, so its offshore wind blows from the north-northeast. Wind tends to be lighter early in the morning, which is one more reason the dawn session is often the best.

In TidePlanet’s surf score, onshore wind starts costing points from about 4.5 mph; offshore wind adds points up to about 20 mph and only takes them away above about 27 mph.

The tide at your break

The tide doesn’t change the size of the swell, but it does change how the waves break, because it changes how deep the water is over the bottom. Some breaks work at low tide, others at high tide, and others on the incoming tide. Where the tide moves several feet, the same beach can go from good to unrideable in a couple of hours.

What to check:

• the times of high and low tide for the day, as we explain in how to read a tide chart;
• whether it’s a spring tide, when the water rises and falls faster and currents in inlets and river mouths run harder;
• which tide your break likes: ask a local surf shop, a surf school or the people in the lineup.

A surf report, step by step

Imagine this forecast for a beach that faces south-southwest, like Huntington Beach:

	Morning	Afternoon
Swell	3 ft · 15 s · SSW	3 ft · 14 s · SSW
Wind swell	1 ft · 6 s · W	2 ft · 7 s · W
Wind	4 mph from the NNE	12 mph from the W
Tide	low at 6:40 am	high at 1:15 pm

1. Size. 3 feet of significant height offshore. With a 15-second period, it breaks bigger than the number suggests: on an open beach, sets of almost 5 feet (1.5 m), about head high.
2. Period. 15 seconds: a long-period groundswell with well-spaced sets and lulls in between.
3. Direction. From the south-southwest onto a beach that faces south-southwest: it comes straight in, so all of it reaches the beach.
4. Wind swell. In the morning, 1 foot at 6 seconds, hardly anything. In the afternoon it builds to 2 feet from the west, mixes with the groundswell and messes it up.
5. Wind. For a beach facing south-southwest, a north-northeast wind is offshore: 4 mph in the morning grooms the waves. By afternoon, 12 mph from the west is blowing in from the ocean at an angle, so expect chop.
6. Tide. Low tide at 6:40 am and high tide at 1:15 pm: the whole morning is an incoming tide.

Bottom line: go in the morning, from first light to late morning. If your break likes low tide, go as early as you can; if it prefers mid tide, around 9 or 10. And if long-period sets of nearly 5 feet are more than you want, pick a beach that faces away from the swell or a sheltered spot that day.

How TidePlanet shows it

Every coastal place with a marine forecast has two tabs for this.

The Surf tab, like surf in Huntington Beach or surf in Santa Cruz, sums all of the above into a score from 0 to 10 for each hour and for the day, with the best time. It shows:

• the surf at the shore, from the average wave to the sets;
• the swell and the wind swell, each with its height, period and direction;
• the wind, labeled offshore, cross-shore or onshore based on the way that stretch of coast faces;
• an hour-by-hour chart with the wind in color (green offshore, amber cross-shore, red onshore) and the tide below;
• the energy of the swell, the wetsuit for the water temperature, the next 7 days and the months with the most waves.

On U.S. beaches covered by an NWS surf zone forecast, like surf at Nags Head on the Outer Banks, the Surf tab also shows the official NWS surf height and rip current risk.

The Waves tab gives you the raw marine forecast: the significant wave height, the sea state on the Douglas scale, the period, the swell and “the waves you may find”: the most common, the significant and the highest to expect.

The score is an estimate for that stretch of coast, not for a specific break. The wave model sits offshore and doesn’t know the bottom of each beach or which tide it likes. Use it to pick the day and the time, and check the surf when you get there.

Before you paddle out

• Watch the ocean for a few minutes before going in: see how often the sets come and where they break.
• Pick waves at your level. If in doubt, stay on the beach or go with a surf school.
• Follow the flags and the lifeguards.
• Watch for rip currents. If one pulls you out, don’t swim against it: swim parallel to the shore and raise an arm for help.
• Get out of the water in a thunderstorm.

If you’ll also be walking out on rocks or flats at low tide, read up on tide safety before you go.