86% of tide predictions
within 30 minutes

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 model with full-earth coverage, used wherever there is no NOAA or DFO station. Sub-hour timing comes from parabolic interpolation of the hourly output, plus a fixed 30-minute correction: the model's hourly values run half an hour early against NOAA at every station we check.

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 at 16 US tide stations, including Hawaii, Alaska and Puerto Rico. Daily results have been stored since 27 October 2026; the earlier days in the trend were reconstructed afterwards from archived model output, using the same method. Any station below 50%, or an overall score below 60%, triggers an automatic alert.