How to Read a Surf Forecast (Without Getting Fooled by the Numbers)

A surf forecast is a handful of numbers trying to describe something that is never quite the same twice. Learn to read those numbers well and you stop paddling out into disappointment. Read them badly and you will skip the best session of the week because a website gave it two stars.

We read forecasts every single morning for the point in front of our house. Here is how we do it, number by number, in plain words.

Looking for today’s conditions rather than the theory? Our live Medewi surf report shows swell, wind, tide and water temperature in real time. And if you used to check Magicseaweed: it is no longer online, and the old address now redirects to Surfline.

1. Swell height: the ocean, not the wave

The height on a forecast is the height of the swell out in open water, not the face of the wave you will ride. What reaches the beach depends on the spot. An exposed reef can turn a modest swell into something much bigger. A sheltered point or bay can show a lot less than the number suggests.

So treat swell height as a starting point, not a promise. After a few sessions at the same spot you will learn its personality: how big it tends to be compared with the forecast, and at what size it starts to work or starts to close out.

2. Swell period: the most underrated number

Period is the time in seconds between one wave crest and the next. It tells you how much energy a swell carries, and it is the number beginners most often ignore.

A rough guide:

• Under eight seconds: wind swell, made by local wind. Weaker, choppier, often disorganised.
• Around ten to twelve seconds: a proper groundswell. More power, cleaner lines, better shape.
• Thirteen seconds and up: a long-travelled swell with serious energy. The sets arrive bigger than the height suggests, and the gaps between them are longer.

That is why a 1.2 metre swell at 12 seconds usually beats a 1.5 metre swell at 7 seconds. Same ocean, very different waves.

3. Swell direction: does it reach your spot?

Direction tells you where the swell is coming from. Every spot has a window of directions it can see, and swell from outside that window barely arrives. Medewi, for example, loves southwest swell, which wraps neatly around the point and lines the waves up down the reef. The same swell might miss a beach that faces another way.

When a forecast looks good but the spot is flat, direction is usually the reason.

4. Wind: the number that decides the day

You can have perfect swell and still have bad surf if the wind is wrong. Two things matter: where it blows from, and how strong it is.

Offshore wind blows from the land towards the sea. It holds the waves up, smooths the face and makes them peel cleanly. This is what you want. Onshore wind blows from the sea onto the beach and crumbles the waves. Cross-shore sits in between, and can be fine if it is light.

As for strength: very light wind is almost always good, a moderate breeze starts to put texture on the water, and strong onshore wind usually ends the session. In Bali the wind tends to be lightest in the early morning, which is the whole reason surfers here set their alarms so early. In the dry season, the southeast trade winds blow offshore at Medewi, and the mornings are at their cleanest.

5. Tide: the daily timetable

The tide does not show up in a star rating, and yet at many spots it matters as much as the swell. Some waves only work at high tide, some only at low, and some change character completely in between.

At Medewi, mid to high tide is the friendliest: the wave is softer, slower and deeper over the boulders. On a lower tide it gets faster and more critical, and the walk in and out needs more care. We go into more detail in our complete guide to surfing Medewi, and longboarders will find their ideal window in longboarding in Medewi.

Putting it together: two example mornings

Here is how we would read two imaginary forecasts for Medewi.

Morning one: 1.3 metres at 12 seconds from the southwest, light southeast wind, high tide at nine. Every number points the same way. Groundswell from the right direction, offshore wind, friendly tide. Be in the water early.

Morning two: 1.6 metres at 7 seconds, strong onshore wind, low tide. The height looks bigger, but the period is short, the wind is wrong and the tide is at its trickiest. This is a morning for coffee and a waterfall.

Notice that the second morning has the bigger wave height. That is the most common trap in forecast reading.

Why the star rating is not enough

Most forecast sites boil everything down into a single score. Ours does too: the rating on our Medewi surf report is an automatic estimate based on swell height, period and wind. It is a useful shortcut, but it cannot know how a particular sandbar or reef is behaving this week, or that the wind drops for an hour after sunrise.

So use the rating to decide whether to look, and your eyes to decide whether to surf. Always check the conditions in person before paddling out.

The shortcut: surf with someone who reads it every day

Forecast reading is a skill, and like any skill it gets better with a teacher. Part of every coached session with us is understanding why we paddled out when we did and where we sat. After a week, most guests read the ocean differently. If you are ready for that step, have a look at our intermediate coaching, or read how the move to a point break changes things in your first point break.

Quick answers

What does a 12 second swell mean? It means 12 seconds pass between one wave crest and the next. That is a proper groundswell with good energy and clean lines.

What is a good swell period for surfing? Around ten seconds and up is generally good. Under eight seconds is usually weak, choppy wind swell.

What is offshore wind? Wind blowing from the land towards the sea. It holds waves up and makes them clean. Onshore wind does the opposite.

Is swell height the same as wave height? No. Swell height is measured in open water. The waves at a spot can be bigger or smaller depending on how exposed it is.

What is the best tide for Medewi? Mid to high tide, when the wave is softest and most forgiving.

Is Magicseaweed still working? No. Magicseaweed is no longer online and the address redirects to Surfline. For Medewi, our own live surf report is free to use.

Still not sure what the numbers are telling you? Ask us. We read them every morning anyway.