Legal · Data Policy

Data accuracy

Tide predictions in AlaskaTides.app are mathematical forecasts derived from historical measurements. This page explains how predictions relate to observed water levels, why differences can occur, and how to interpret gaps or outages.

Source
NOAA
Official CO-OPS predictions
Datum
MLLW
Mean Lower Low Water
Precision
0.1 ft
Displayed rounding
Navigation
NOT approved
Planning purposes only
Predictions vs. observations
Predicted tides
Mathematical
Harmonic constituents from historical data
Derived from long records of gauge measurements. Consistent and deterministic — the same numbers every year for the same station.
Observed water levels
Real-time
Live NWLON gauge readings
What the water is actually doing, incorporating storm surge, wind setup, atmospheric pressure, and river flow.
Sources of difference
Wind & storm surge
Strong onshore winds push extra water against the coast. Offshore winds lower levels. Can add or subtract 2–4+ feet in extreme Aleutian or Bering storms.
Atmospheric pressure
Low pressure raises water (~1 cm per hPa). The "inverse barometer" effect is significant during deep Aleutian Low passages.
River flow & snowmelt
Freshwater inflow in spring can raise levels at tidal river mouths and deep inlets beyond what astronomical tides predict.
Ice conditions
Sea ice and river ice can modify how the tide signal propagates — particularly in Western and Arctic Alaska.
Sparse records
Predictions for historical and subordinate stations are based on shorter records. Errors can be larger and less predictable than at NWLON sites.
Local bathymetry
Complex seafloor geometry — like upper Cook Inlet's funnel shape — amplifies tidal signals beyond what simple modeling captures.
Gaps, outages, and time zones

Outages: NOAA APIs and individual stations sometimes go offline temporarily. When live data is unavailable, AlaskaTides shows predicted values only and labels data accordingly.

Time zones: Predicted times are displayed in local time for the station where possible, including daylight saving adjustments. Minor rounding differences from raw numerical values may appear.

Station changes: NOAA periodically updates harmonic constituents, datum values, and supported products. AlaskaTides aims to reflect current predictions but cannot guarantee continuous availability for every station.

Use for planning only
⚠️
Not approved for navigation
AlaskaTides.app is intended for general planning purposes only. For safety-critical decisions, always consult official charts, local notices to mariners, current observations, and on-the-water conditions.
Marine safety disclaimer → About stations → NOAA official ↗