Methodology

Data last computed 2026-07-04. This page describes the exact pipeline.

The window

A tide window is a contiguous interval when the predicted water height at a NOAA station sits below +1.0 ft MLLW — a threshold at which most Pacific tidepool shelves and low-tide beaches become comfortably walkable. Heights come from NOAA CO-OPS harmonic tide predictions (datum MLLW, feet), fetched fresh every day for the next ~400 days.

• Harmonic stations: we fetch the hourly prediction series and linearly interpolate the exact crossing times of +1.0 ft, rounded to 5 minutes.
• Subordinate stations (high/low events only): we use standard cosine interpolation between consecutive extremes to estimate crossings. Where both methods can be compared at harmonic stations, they agree within about ±15 minutes.

Arrive by is 60 minutes before the low — the classic advice for working out to the lowest zone as the water still falls.

The 0–100 score

• Depth (0–50): linear from 0 points at +0.5 ft to 50 points at −2.0 ft (clamped). Lower tide, more exposed seafloor.
• Daylight (0–30):the window's overlap with computed sunrise–sunset, scaling linearly to 30 points at 3 hours. Windows with under 30 minutes of daylight score 0 total and are labeled night tides. Sun times are computed with the open-source astronomy-enginelibrary for the station's exact coordinates.
• Timing (0–10): Saturday, Sunday, or US federal holiday +10; Friday +5.
• Season comfort (0–10): a fixed, published table by region and month — a simple heuristic for daylight length and typical shoulder-season comfort. It is deliberately coarse and is not a weather forecast.

Bands: 90–100 Exceptional · 75–89 Great · 60–74 Good · 40–59 Fair · under 40 Skip.

Enrichment data

• Short-range conditions (windows within 7 days only): NWS point forecasts from api.weather.gov, stamped with their fetch date.
• “What's in the pools”: research-grade observation counts within 5 km over the last 60 days from the iNaturalist open API (© contributors, CC BY-NC), filtered to molluscs, echinoderms, cnidarians, and arthropods.

Honest limitations

• Predictions are astronomical. Wind, swell, and pressure routinely shift actual water levels by half a foot or more — check conditions before you go, and never turn your back on the ocean.
• A station's predictions apply exactly at the station. Nearby beaches can lag or lead by tens of minutes; we name the station on every page so you can judge the distance.
• The walkable threshold varies by beach profile; +1.0 ft is a defensible default, not a law of nature.

Automation disclosure

Tidewindow is an automated publication: the data pipeline recomputes every number daily, and the editorial content is produced by an AI system operating under strict rules — every tide statistic must come from the computed dataset (never from a language model's memory), factual claims must cite a source fetched at publish time, and safety guidance is only ever quoted from park services. Full details on the about page. Found an error? Tell us — corrections ship within a day.