Every long-period ground swell on the planet starts the same way: a storm somewhere in the open ocean blows wind across the water for long enough, hard enough, and over a long enough fetch that it generates waves that survive the journey to your beach. Understanding the geometry of storm-driven swells turns weather maps from background noise into a tool you can use.
The three ingredients
Wave-generating storms need three things working together:
- Wind speed: the higher the better. 30+ kt sustained winds produce serious swells; 50+ kt produce the kind of legendary winter swells that fire Mavericks or Pipeline.
- Fetch: the over-water distance the wind blows in roughly the same direction. Long fetch + sustained winds = bigger, longer-period swells. A 1,000+ nm fetch is what separates Pacific ground swells from East Coast hurricane swells.
- Duration: how long the wind blows. 12–24 hours of sustained 40 kt winds will fully develop the sea state for that wind speed; beyond that, you can't grow the swell any more (the "fully developed sea" limit).
What happens when the storm ends
Once the wind stops, the waves decay — they lose energy slowly as they propagate outward. But not all wavelengths decay at the same rate. Short-period (high-frequency) components dissipate first; long-period (low-frequency) components keep their energy for thousands of miles. This is why a Gulf of Alaska storm produces 14 s ground swells at the North Shore three days later, even though the storm was 2,500 nm away.
The swell field that arrives at your beach is the dispersive tail of the storm: long periods first, then progressively shorter periods. A buoy watching a remote swell approach will see the period drop over time as the storm's full spectrum arrives.
Group velocity and travel time
Waves propagate at their group velocity: cg ≈ 0.78 · T m/s in deep water. A 14 s wave travels at ~10.9 m/s ≈ 39 km/h. So:
- A storm 1,000 nm away producing a 14 s swell will arrive in ~48 hours.
- A storm 2,000 nm away takes ~96 hours.
- A storm 4,000 nm away takes ~190 hours (8 days).
This is why surf forecasters watch storm systems thousands of miles offshore — the energy is already on the way, you just have to wait. The session score on this site uses group velocity to estimate the origin distance for arriving long-period swells.
East Coast vs West Coast storm patterns
East Coast: two main storm types feed surf.
- Hurricanes (Aug–Nov): tropical systems generate clean E and ESE swells that travel up the seaboard. The best East Coast surf often comes from hurricanes 500–1,500 nm offshore — close enough to deliver size, far enough to deliver clean lines.
- Nor'easters (Oct–Apr): extratropical cyclones up the Atlantic coast generate big NE swells. Often storm conditions at the same time as the surf — onshore winds, rain, cold water — but the swells themselves are big and long-period.
West Coast: three main sources.
- North Pacific winter storms (Nov–Mar): Aleutian and Gulf of Alaska lows generate W and NW swells. Travel 2,000–3,000 nm to reach California.
- Southern Hemisphere swells (May–Sep): winter storms in the Southern Ocean generate S and SW swells that travel up the Pacific. Clean, long-period, but small.
- Tropical systems (occasional): Eastern Pacific hurricanes produce S swells for Southern California.
The narrative this site builds
The swell narrative panel on every forecast page identifies the dominant swell type, estimates its origin distance using group velocity, and flags whether the swell is building, holding, or fading over the next 24 h. It pulls all this from the wave model output you'd otherwise have to dig into yourself.
Practical use
- When you see "ground swell from the NE at 14 s, originating ~2,500 km away" — that means there's a storm in the central Atlantic, the swell is 2–3 days into a 3-day journey, and the peak is still ahead.
- Long-period swells are direction-fragile: 14 s at 30° off beach-normal arrives at full power; 14 s at 75° off arrives at half power.
- Short-period swells (under 9 s) are usually within 12–24 h of being generated. They're not from a distant storm — they're from this morning's wind, somewhere nearby.
- Watch the period trend on the wave chart. Rising period over hours = an approaching swell from a distant storm. Falling period = the swell train has passed through its peak.