Big Island Swell Science 101
Reading the Ocean's Mood and Mastering the Forecast
Written by a Local Surf Expert
Kalani MillerUnderstanding Why Waves Break the Way They Do
Understanding why the waves are breaking the way they are is the next step in becoming a true waterman or waterwoman. The Big Island's surf is shaped by unique forces that make it different from any other place in the world.
This is the most important concept for understanding our waves. Look at a map of the Hawaiian Islands. The Big Island sits to the southeast of Maui, Molokaʻi, and Oʻahu.
The Great Swell Shadow
During the winter, powerful storms in the North Pacific generate huge Northwest swells (NW) that march down toward Hawaiʻi. These are the swells that create the legendary big waves on Oahu's North Shore. But before they can reach our Kona coast, they are blocked by the other islands. Think of Maui as a giant bodyguard for Kona, taking the full force of the swell. Only a fraction of that energy "wraps" around the other islands and filters into our leeward shores.
This "swell shadow" is why a 20-foot day on Oahu might only be a 4-6 foot day in Kona. It makes our winter surf more manageable but also less consistent than on the other islands. This effect is a key reason why high-resolution wave models are so valuable for forecasting here, as they can help predict this complex shadowing.
❄️ Winter (November - April)
This is the season of the North Pacific. Powerful storms generate those big NW swells. These swells travel unimpeded across the ocean and slam directly into the Hilo and Hāmākua coasts on the island's windward (east) side. This is when spots like Honoliʻi get big and powerful.
- • Hilo coast: Big and powerful
- • Kona coast: Smaller, wrapped energy
- • WNW swells can still produce fun waves
☀️ Summer (May - October)
This is Kona's prime time. The storm track shifts to the Southern Hemisphere. Massive storms off the coasts of New Zealand and Antarctica generate long-period South swells (S) that travel thousands of miles north across the Pacific. These swells march straight into the Kona coast with nothing in their way.
- • Kona coast: Consistent, clean waves
- • Hilo coast: Mostly flat
- • Classic breaks come alive
The All-Important Wind
Wind is the final ingredient that determines wave quality.
🌬️ Tradewinds (NE)
These are the prevailing winds in Hawaiʻi, blowing from the northeast. For the Kona coast, these winds are perfectly offshore, blowing against the face of the incoming waves, holding them up, and grooming them into clean, smooth lines. This is why Kona mornings are famously glassy. For the Hilo coast, these same winds are onshore, which can make conditions choppy and disorganized.
🌪️ "Kona" Winds (SW)
Occasionally, the weather pattern shifts, and we get winds from the southwest. These are called "Kona" winds because they are onshore and messy for the Kona coast. But for Hilo, these winds are pure gold. A rare Kona wind day combined with a solid NE swell creates perfectly groomed, offshore conditions at Honoliʻi—a magical combination that every Hilo surfer dreams of.
🧭 Swell Directions
- North (N) Hilo coast
- Northwest (NW) Winter swells
- South (S) Kona coast
- Southwest (SW) Summer swells
💨 Wind Effects
Clean, groomed waves
Choppy, disorganized
Textured, workable
📖 Surfing Guide
Your Forecasting Toolkit
To put this knowledge into practice, use the same tools the pros use. I recommend checking Surfline for its detailed spot forecasts, live cams, and daily analysis. For a more technical look at the swell models, including the island shadowing effects, the Pacific Islands Ocean Observing System (PacIOOS) provides incredible regional wave model data developed right here at the University of Hawaiʻi.
Surfline
- • Detailed spot forecasts
- • Live webcams
- • Daily analysis
- • User-friendly interface
- • Mobile app available
PacIOOS
- • University of Hawaiʻi data
- • Regional wave models
- • Island shadowing effects
- • Technical/scientific approach
- • High-resolution forecasts
NOAA/NWS
- • Official weather service
- • Marine forecasts
- • Wind predictions
- • Storm tracking
- • Safety advisories
Windy.com
- • Wind pattern visualization
- • Multiple weather models
- • Interactive maps
- • Storm tracking
- • Free to use
Key Elements to Check
Swell
- • Direction (compass)
- • Size (feet)
- • Period (seconds)
- • Consistency
Wind
- • Direction (compass)
- • Speed (knots/mph)
- • Timing (morning/afternoon)
- • Consistency
Tides
- • High/low times
- • Tide height
- • Rising/falling
- • Reef exposure
Local Forecasting Pro Tips
Kona Coast
- • Best surf: May-October
- • Look for S/SW swells
- • Dawn patrol for glass-off
- • Check swell period (12+ seconds better)
- • NW swells can wrap in winter
Hilo Coast
- • Best surf: November-April
- • Look for N/NE swells
- • Rare Kona winds = magic
- • Usually onshore trades
- • River runoff after rain
What About Flat Days?
Even in paradise, the ocean needs to rest. Discover amazing alternatives when the surf goes flat.
Explore Flat Day Adventures