How to Read a Surf Forecast: Swell, Period, Wind and Tide

Surf forecasts look like a wall of numbers and arrows: "4 ft @ 14 s SW, wind E 8 kt, low 6:15." Once you know what each part means and how they interact, you can look at a forecast and know roughly whether it's worth going, when, and which break to choose. This guide walks through each element, then shows you how to put them together.

Swell height: what the number actually measures

Most forecasts lead with a swell or wave height. Two things to know:

• It's a statistical average, not the biggest wave. Buoy and model "significant wave height" is roughly the average of the highest one-third of waves, as NOAA's National Data Buoy Center explains. Some waves in a set will be noticeably bigger.
• Swell height is measured offshore, not at your beach. What reaches your break depends on period, direction and the shape of the seafloor. Forecast sites that show "surf height" have already estimated how the swell will break. Sites that show raw swell height leave that to you.

Forecasts usually list more than one swell: a primary swell plus one or more secondary swells from other directions. The National Weather Service notes that multiple coexisting wave groups are common, and each can matter differently. A secondary swell can add peaks and variety, or it can make the surf confused and lumpy.

Swell period: the number that tells you about power

Period is the time, in seconds, between successive wave crests passing a fixed point. According to the National Weather Service, it relates to how fast waves travel, how deep their energy reaches, and how much energy they carry, all of which affects how big they break at the coast.

Short-period swell is generated by nearby wind and arrives disorganized. Long-period swell (often called groundswell) was born in a distant storm and has had time to organize into clean, powerful lines.

Period	Type	What to expect
5–8 s	Wind swell	Short, choppy, weak waves; can still be fun at exposed beach breaks
9–12 s	Medium period	Waves organizing into sets; decent surf at well-exposed spots
13–16 s	Groundswell	Clean, well-spaced, powerful waves; often the sweet spot
17 s +	Long-period groundswell	Very powerful, often bigger than forecast, longer lulls, strong currents

These bands are widely used by surf forecasters, though the exact cutoffs vary from source to source. The practical takeaway is that a 3 ft swell at 15 seconds will usually produce better (and bigger) surf than a 5 ft swell at 7 seconds.

Swell direction and your break's swell window

Direction is where the swell comes from, given as a compass point or in degrees. A "SW" swell travels from the southwest toward the northeast.

Every break has a swell window, the range of directions it's open to. A beach facing west might light up on a W or NW swell and stay nearly flat on a S swell blocked by a headland. That's why one beach can be pumping while the next cove is flat. Learning which directions work at your local spots is one of the most useful things you can do as a surfer.

• Exposed beach breaks catch swell from a wide range of directions.
• Points and coves often need a specific direction to wrap in.
• Longer-period swells bend (refract) more around headlands and reefs, so they can reach spots that short-period swell can't.

Wind: the make-or-break factor

Swell creates the waves; local wind decides their shape.

Wind	Direction	Effect on surf
Offshore	Land to sea	Grooms the faces, holds waves up, clean conditions. Light offshore is ideal; strong offshore can make paddling into waves hard.
Glassy / calm	No wind	Smooth, mirror-like faces; most common early morning
Cross-shore	Parallel to the beach	Uneven texture; manageable when light, messy when strong
Onshore	Sea to land	Chop, crumbly waves that break early and close out; strong onshore ruins good swell

Watch the timing, too. On many coasts, a morning land breeze (offshore) gives way to an onshore sea breeze by late morning or midday. That's why "dawn patrol" sessions are a surfing tradition. A forecast that says "offshore until 10, then onshore 15 kt" is telling you to go early.

Wind speed is usually given in knots. Light winds of roughly 5–15 knots offshore are generally considered ideal. Anything strong and onshore is usually a write-off for quality surf.

Tide: why the same beach changes all day

NOAA explains that most coastal areas get two high tides and two low tides in each lunar day of 24 hours and 50 minutes. That's why tide times shift roughly 50 minutes later each day. As the water level rises and falls over a reef or sandbar, the same swell can break completely differently.

• Low-tide spots need shallow water over sandbars to make waves break. At high tide they can go fat and slow.
• Mid-to-high tide spots may close out or get dangerously shallow at low tide over reef or rock.
• Tide changes often switch a spot on or off, sometimes within an hour.
• Tidal range matters. The bigger the difference between high and low water on your coast, the more dramatically a spot changes through the day.

There's no universal "best tide." Ask locals, check spot guides, and keep notes on when your break works.

Putting it together: a worked example

Here's a sample forecast for a west-facing beach break:

• Primary swell: 4 ft @ 14 s from SW
• Secondary swell: 2 ft @ 8 s from W
• Wind: E (offshore) 8 kt until 10 a.m., then SW (onshore) 15 kt by noon
• Tide: low 6:15 a.m., high 12:45 p.m.

How to read it:

1. Swell: 4 ft at 14 seconds is a solid groundswell, and it may break larger than 4 ft at an exposed beach. The short-period W swell will add some texture.
2. Direction: SW suits this beach's exposure.
3. Wind: Clean, offshore conditions early. The onshore sea breeze will blow it out by midday.
4. Tide: Low early, filling toward mid-tide by around 9:00 a.m.
5. Verdict: The best window is roughly 8:00–10:00 a.m., with good swell, offshore wind and a rising mid-tide. After that, it's done.

Safety checks every forecast needs

Before you paddle out, look past the swell chart:

• Rip current risk: The National Weather Service issues Surf Zone Forecasts for many coastal areas, with a rip current outlook of low, moderate or high. Even on a "low" day, life-threatening rips often occur near groins, jetties, reefs and piers. On a "high" day, they're likely.
• High surf advisories: If your local NWS office has issued one, treat it as a clear signal to stay on the beach unless you're an experienced surfer who knows that break.
• Lifeguards: Ask about the day's hazards when you arrive. They know which channels are pulling.
• Water temperature: Check it and dress accordingly (see our wetsuit temperature guide).
• Lightning and storms: Thunder means out of the water. See Reading Weather and Lightning Risk on the Water.

Just starting out? Pair this with Your First Surfboard and Surf Etiquette: The Unwritten Rules of the Lineup.

FAQ

What is a good swell period for surfing?

Roughly 10 seconds and up starts to produce organized surf, and about 13–16 seconds is often considered the sweet spot for clean, powerful waves. Very long periods (17 seconds and up) can be powerful and inconsistent.

Is offshore wind good for surfing?

Yes, when it's light. It smooths and holds up the wave faces. Strong offshore wind can make it hard to paddle into waves and blows spray back at you. Onshore wind generally makes the surf choppy and messy.

Why is the surf smaller than the forecast?

The swell may come from a direction your break doesn't receive well, the period may be short, the tide may be wrong for that spot, or the forecast height may be offshore swell rather than breaking surf height.

What's the best tide for surfing?

It depends entirely on the spot. Some breaks work best at low tide over sandbars, and others need mid to high tide to cover reef or rock. Learn your local break's preference over time.

Sources

• NOAA National Data Buoy Center — How are significant wave height, dominant period, average period, and wave steepness calculated?
• National Weather Service — Coastal Waters Forecasts With Wave Detail
• National Weather Service — NWS Offices Issuing Surf Zone Forecasts
• NOAA National Ocean Service — Frequency of Tides: The Lunar Day
• Rapture Camps — Surf Forecast Basics: How to Read Swell, Wind, and Tide Reports