Thornton State Beach

Faces W · Bluff Access · Unstable Cliffs · No Lifeguard

Current Danger Level

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Buoy Reading — View Buoy ↗

⚠ Data may be stale — showing last known reading. Conditions may have changed.

Swell Height*

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feet

Swell Period

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seconds

Direction

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swell from

Water Temp

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°F

NOAA Buoy 46026 — San Francisco Bar

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Long-Period Swell & Set Waves
Read the table above. Any swell period above 15 seconds means danger. Waves travel in sets separated by long, quiet intervals — sometimes as long as 20 minutes of calm. On long period days, stay aware! Do not approach the water's edge during a quiet stretch — that calm is when energy is building offshore.

Known Hazards at Thornton State Beach

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Active Cliff Erosion

The bluffs above Thornton Beach are actively eroding and collapse without warning. Stay clear of the cliff face at all times. Do not stand beneath or near the bluffs. Fatalities have occurred at this beach.

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Rock & Debris on Beach

Fallen rock and eroded material create unstable surfaces. Combined with wave surge, footing near the water is dangerous.

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Cold Water — Year Round

Pacific water along this coast runs cold year round. Hypothermia can set in quickly.

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No Year-Round Lifeguard Posted

CA State Parks lifeguards patrol state beaches. There is no year-round posted lifeguard. Emergency response times on remote beaches may take longer than in urban environments.

Before You Go Near the Water

Read the swell period before you approach the water. The number in the chart above is your warning. A period of 15+ seconds means sets are arriving with long, quiet intervals between them — sometimes up to 20 minutes of calm. Do not approach the water's edge during a quiet stretch — that calm is when energy is building offshore.

Watch the ocean. Stay on dry sand. Do not approach wet sand or the water's edge until you have watched the ocean and learned the rhythm of the sets.

Wet rocks or sand mark where the ocean has already been. If the rocks or sand are wet, a wave reached them recently. Stay above the wet line — always.

A long wait between waves is a warning. During long period swells, stay vigilant. The ocean may appear completely calm between sets. Do not be fooled. If it has been calm for several minutes, stay back. Do not walk toward the water.

Keep dogs on leash. Keep dogs on leash and away from the water's edge.

Tell someone your plans. Leave your destination and expected return time with someone before you go.

How to Read Sets

Long-period swell doesn't produce a steady stream of waves. It travels in sets — groups of larger waves separated by long, quiet intervals. At 15+ seconds, those intervals can stretch to 20 minutes of calm. Do not approach the water's edge during a quiet stretch — that calm is when energy is building offshore.

Find a high vantage point above the water line. Do not approach the shore first. Watch from above.

Time the quiet intervals. Count the seconds — or minutes — between large waves. This is your set interval.

Watch for at least 15 minutes. You need to see multiple sets before you understand the rhythm. One quiet stretch means nothing.

The largest wave in a set is not always the first. Sets build in size. The final wave is frequently the biggest.

When in doubt, stay back. At 15+ seconds period, stay completely clear of the shore and any rocks.

Ocean Emergency

911

Daly City Fire · (650) 991-8100

CA State Parks Emergency · (800) 952-5580

USCG San Francisco · (415) 399-3547