Understanding NOAA Tide Predictions: A Complete Guide

Tide predictions are remarkably accurate—typically within 6 inches and 15 minutes—but they're predictions, not observations. Understanding how they're calculated and what affects them helps you use tide data more effectively.

How Tide Predictions Work

The Basics of Tidal Forces

Tides are caused by the gravitational pull of the moon and sun on Earth's oceans:

1. Moon's Influence: About 2x stronger than the sun (it's closer)
2. Sun's Influence: Adds to or subtracts from the moon's pull
3. Earth's Rotation: Creates the twice-daily tide cycle
4. Local Geography: Amplifies or dampens the astronomical tide

Harmonic Analysis: The Science Behind Predictions

NOAA uses harmonic analysis to predict tides. This mathematical method:

1. Analyzes Historical Data: Years of water level measurements
2. Identifies Patterns: Breaks tides into predictable cycles (constituents)
3. Calculates Coefficients: Determines each cycle's strength at that location
4. Predicts Future: Combines all cycles to forecast future tides

Think of it like predicting sunrise times—it's based on orbital mechanics that don't change.

Tidal Constituents

Tide predictions use dozens of harmonic constituents, each representing a different astronomical cycle:

Primary Constituents:

• M2 (Principal lunar): 12.42 hours - The main twice-daily tide
• S2 (Principal solar): 12.00 hours - Solar influence
• N2 (Lunar elliptic): 12.66 hours - Moon's varying distance
• K1 (Lunar diurnal): 23.93 hours - Once-daily component
• O1 (Lunar diurnal): 25.82 hours - Moon's declination

How They Combine: When sun and moon align (new/full moon) = Spring tides (higher highs, lower lows) When at right angles (quarters) = Neap tides (less extreme)

Understanding Tide Tables

Key Terms

High Tide: Maximum water level during the tidal cycle Low Tide: Minimum water level during the tidal cycle Tide Range: Difference between high and low tide Spring Tides: Highest high tides and lowest low tides (new/full moon) Neap Tides: Smallest tide range (quarter moons)

Reading a Tide Table

Example from San Francisco:

Date       Time      Height    Type
Feb 14    2:15 AM    -0.5 ft    Low
Feb 14    8:30 AM     5.8 ft    High
Feb 14    2:45 PM     0.2 ft    Low
Feb 14    9:10 PM     4.9 ft    High

What This Tells You:

• Two high tides and two low tides (semidiurnal pattern)
• Morning high is higher (5.8 ft vs 4.9 ft)
• Morning low is lower (-0.5 ft vs 0.2 ft)
• Tide range: 6.3 ft (morning) and 4.7 ft (evening)

Time Zones

All times on HighTideNearMe are shown in the local time zone for that station. This makes planning easier—no conversion needed!

Tide Datums: Understanding the Reference

Mean Lower Low Water (MLLW)

All tide heights are measured relative to MLLW—the average of the lowest tide each day over 19 years.

Why MLLW?

• Ensures water depths shown on charts are minimum expected
• Safe for navigation (actual water usually deeper)
• Standard reference point

Other Common Datums:

• MSL (Mean Sea Level): Average water level
• MHHW (Mean Higher High Water): Average of highest high tides
• MLW (Mean Low Water): Average of all low tides

Example Heights at San Francisco:

• Highest tide: +7.0 ft above MLLW
• Sea level (MSL): ~3.0 ft above MLLW
• Lowest tide: -1.9 ft below MLLW (yes, negative!)

Why Predictions Differ from Reality

1. Weather Effects

Atmospheric Pressure:

• Low pressure (storms): Water level rises ~1 inch per millibar
• High pressure: Water level drops
• Effect can be 6-12 inches

Wind:

• Onshore winds: Push water toward shore (higher than predicted)
• Offshore winds: Push water away (lower than predicted)
• Strong winds: Can add/subtract 1-3 feet

Storm Surge:

• Hurricanes/nor'easters: Can add 5-20+ feet
• Combines with high tide for extreme levels
• Not included in predictions

2. Seasonal Variations

Thermal Expansion:

• Warmer water = higher sea level
• Summer: +2-4 inches above average
• Winter: Lower levels

Ocean Currents:

• El Niño: Higher water on West Coast
• Changes in Gulf Stream affect East Coast
• Seasonal current shifts

River Runoff:

• Spring snowmelt raises water levels
• Heavy rain increases bay/estuary levels
• Can affect tide timing slightly

3. Long-term Sea Level Changes

Sea Level Rise:

• Currently ~3mm/year globally
• Varies by location
• Predictions don't account for future rise
• Higher "normal" high tides over time

Accuracy of Predictions

Under Normal Conditions

Height Accuracy: Within 6 inches (15 cm) Time Accuracy: Within 15 minutes

Factors Affecting Accuracy

Very Accurate (±3 inches, ±5 minutes):

• Open coast stations
• Deep water
• Stable weather
• Far from rivers

Less Accurate (±12 inches, ±30 minutes):

• Shallow bays
• River estuaries
• During storms
• Complex coastlines

Example Accuracy:

• San Francisco: Excellent (open bay)
• Chesapeake Bay tributaries: Good
• Louisiana bayous: Fair (river effects)

Different Tide Patterns

Semidiurnal (Most Common)

Two high tides and two low tides per day, roughly equal in height.

Locations:

• Atlantic Coast
• Most of Pacific Coast
• Gulf Coast (eastern)

Example: Boston, MA

• High, low, high, low every ~12.4 hours

Mixed (West Coast Pattern)

Two high and two low tides, but with significant height differences.

Locations:

• California
• Oregon
• Washington
• Alaska

Example: San Francisco, CA

• Higher high tide: 6.2 ft
• Lower high tide: 4.1 ft
• Lower low tide: -1.2 ft
• Higher low tide: 0.8 ft

Diurnal (Rare)

One high tide and one low tide per day.

Locations:

• Northern Gulf of Mexico
• Some Pacific islands

Example: Pensacola, FL

• One tidal cycle per ~24.8 hours

Using Predictions Effectively

For Boating

Check These:

1. Low tide time (for navigating shallow water)
2. Tide height at departure/arrival
3. Current predictions (separate from tide height)
4. Weather forecast (affects actual levels)

Add Safety Margin:

• Plan for 1 foot less water than predicted
• Account for boat draft plus propeller
• Watch for weather changes

For Beach Activities

Best Times:

• Swimming: Mid to high tide (more water, less exposed hazards)
• Beach walking: Low tide (more beach exposed)
• Tidepooling: 1-2 hours on either side of low tide
• Beach combing: Falling tide (new items exposed)

For Photography

Use Tide Data to Plan:

1. Check tide schedule for location
2. Combine with golden hour calculator
3. Scout location at similar tide level
4. Plan backup shots for different conditions

High Tide = Drama:

• Waves crashing
• Water features active
• Narrower compositions

Low Tide = Exploration:

• Tidepools
• Exposed rocks
• Wide landscapes

For Fishing

Tidal Movement = Active Fish:

• Best fishing: 2 hours before to 1 hour after high/low tide
• Moving water brings food
• Structure becomes accessible/inaccessible
• Species-dependent strategies

Spring vs. Neap Tides

Spring Tides (Not Related to Season!)

Occur during new and full moons (every ~14 days).

Characteristics:

• Higher high tides
• Lower low tides
• Stronger currents
• Greater range

Best For:

• Dramatic photography
• Exploring low tide areas
• Surfing (bigger waves)
• Observing extremes

Neap Tides

Occur during quarter moons (every ~14 days).

Characteristics:

• Lower high tides
• Higher low tides
• Weaker currents
• Smaller range

Best For:

• Calm conditions
• Learning to boat
• Consistent water levels
• Less dramatic scenes

Advanced: Tidal Currents vs. Tides

Important Distinction:

• Tide Height: How high the water is
• Tidal Current: How fast the water is moving

Current Timing:

• Maximum current: Mid-way between high and low (NOT at high/low)
• Slack water: Around high and low tide (minimal current)

For boating and kayaking, current predictions are often more important than tide height.

Climate Change and Tide Predictions

Current Impact

Sea Level Rise:

• Current predictions based on historical baseline
• Actual high tides now higher than 20 years ago
• Low tides also higher

What This Means:

• "Nuisance flooding" during normal high tides
• King tides cause more flooding
• Old tide predictions slightly low

Future Predictions

With continued sea level rise:

• Today's king tides = future normal high tides
• Areas that occasionally flood = regularly flooded
• Need to add ~1-3 feet to predictions by 2050-2100

Check our king tides article to see future sea levels.

Common Questions

Why are there two high tides per day?

Earth's rotation creates two tidal bulges—one facing the moon, one on the opposite side. As Earth rotates, each location passes through both bulges.

Why do tide times change each day?

The moon orbits Earth every ~24.8 hours, so tides occur about 50 minutes later each day.

Can tides be predicted years in advance?

Yes! Since tides are based on astronomical cycles, they can be accurately predicted decades ahead. Weather effects can't be predicted, but the astronomical tide can.

Why does my location have higher/lower tides than the prediction?

Local weather (wind, pressure), seasonal factors, and unusual events (tsunamis, storm surge) all affect actual water levels.

Are tide predictions getting less accurate with climate change?

The astronomical predictions remain accurate, but sea level rise means the absolute water levels are higher than the historical datum. NOAA periodically updates datums to account for this.

Conclusion

Understanding tide predictions helps you plan coastal activities safely and effectively. Remember:

1. Predictions are highly accurate under normal conditions
2. Weather can significantly affect actual water levels
3. Local geography matters
4. Combine tide data with weather forecasts
5. Add safety margins for critical activities

Find Accurate Tide Predictions: Use HighTideNearMe to access NOAA predictions for 237 coastal stations, updated daily.

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Related Articles:

• What Are King Tides?
• How Tides Affect Coastal Photography
• Data Sources and Methodology

Popular Tide Stations:

• San Francisco, CA - Mixed tide pattern
• Boston, MA - Semidiurnal pattern
• Pensacola, FL - Diurnal pattern