70% of tide predictions
within 30 minutes
30 minutes is the difference between catching the right session and missing it. Verified live against NOAA's official harmonic predictions — 91 tide extremes across 6 open-coast US stations.
How we measure this
Reference: NOAA's official harmonic tide predictions — the same data used by the US Coast Guard, NOAA Weather, and commercial mariners. Derived from decades of measured tidal observations at each station.
Our model: Open-Meteo's NEMO global tidal model — a physics-based ocean circulation model with full-earth coverage, including every surf break that Surfline doesn't reach. Sub-hour timing is resolved via parabolic interpolation of the hourly model output.
The match: For each NOAA predicted high or low tide, we find the nearest Groundswell prediction of the same type and measure the timing difference in minutes. 30 minutes is the meaningful threshold — the difference between catching the right window and missing it.
Daily monitoring: An automated check runs every day, comparing predictions against NOAA reference data across 6 open-coast US stations. Results are stored and used to build the 12-month trend above. Any station falling below 50%, or the overall score below 60%, triggers an automatic alert.
Data Sources
Every number on this site comes from a free, open data source. No proprietary feeds, no paywalled models. Here is exactly where each piece of data originates.
Wave height, swell direction & period, sea surface temperature
Wind speed & direction, air temperature, UV index, precipitation probability, weather codes
Historical surf conditions (back to Jan 2022) and 3-year surf climatology averages
Harmonic tide predictions for US coastal stations; ground-truth reference for our accuracy verification
Harmonic tide predictions for Canadian coastal stations
Tide predictions for all locations outside NOAA / DFO coverage — the rest of the world
Live ocean buoy readings — wave height, dominant period, swell direction, water temperature, wind
Location search and geocoding — converts place names to coordinates
Nearby surf spot names and coordinates — bundled at build time from Surfline's public web endpoint; all forecast data is sourced independently from Open-Meteo
Supplementary nearby spot discovery — fallback for regions not covered by Surfline
Supplementary surf break data (Q693906 surfing break · Q2368508 surf spot) for sparse coverage areas
Interactive map tile backgrounds (light and dark themes)
Land polygon masking — ensures animated swell arcs stop at coastlines on the live map
| Source | Used for | Coverage | License |
|---|---|---|---|
| Wave height, swell direction & period, sea surface temperature | Global oceans | Free · CC BY 4.0 | |
WeatherOpen-Meteo Forecast API ↗ | Wind speed & direction, air temperature, UV index, precipitation probability, weather codes | Global | Free · CC BY 4.0 |
| Historical surf conditions (back to Jan 2022) and 3-year surf climatology averages | Global | Free · CC BY 4.0 | |
TidesNOAA CO-OPS ↗ | Harmonic tide predictions for US coastal stations; ground-truth reference for our accuracy verification | United States | Free · public domain |
| Harmonic tide predictions for Canadian coastal stations | Canada | Free · open govt licence | |
| Tide predictions for all locations outside NOAA / DFO coverage — the rest of the world | Global (fallback) | Free · CC BY 4.0 | |
BuoyNOAA NDBC ↗ | Live ocean buoy readings — wave height, dominant period, swell direction, water temperature, wind | Global (US-operated) | Free · public domain |
LocationOpenStreetMap Nominatim ↗ | Location search and geocoding — converts place names to coordinates | Global | Free · ODbL |
LocationSurfline ↗ | Nearby surf spot names and coordinates — bundled at build time from Surfline's public web endpoint; all forecast data is sourced independently from Open-Meteo | Global | Public web data |
LocationOpenStreetMap Overpass API ↗ | Supplementary nearby spot discovery — fallback for regions not covered by Surfline | Global | Free · ODbL |
LocationWikidata ↗ | Supplementary surf break data (Q693906 surfing break · Q2368508 surf spot) for sparse coverage areas | Global | Free · CC0 |
| Interactive map tile backgrounds (light and dark themes) | Global | Free · attribution required | |
| Land polygon masking — ensures animated swell arcs stop at coastlines on the live map | Global | Free · public domain |
Open-Meteo, NOAA, DFO, CartoDB, OSM, and Natural Earth are used under their respective open licenses. Surfline spot names and coordinates are bundled at build time from Surfline's public web endpoint for nearby spot discovery only; all forecast data is sourced independently from Open-Meteo.