San Mateo County · Half Moon Bay
Cowell Ranch
State Beach
Remote · Rocky Shore · No Lifeguard · No Cell Service
⚠ Data may be stale — showing last known reading. Conditions may have changed.
Wave 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 46012 — Half Moon Bay
Long-period swell is dangerous. Any swell period above 15 seconds means waves are arriving in sets with long quiet intervals — sometimes 20 minutes of calm. During the quiet, energy is building offshore. People who walk to the water's edge during the lull are swept in when a set arrives.
Known Hazards — Cowell Ranch
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! During the quiet periods between sets, energy is building offshore. People who walk to the water's edge during the lull are swept in when a set arrives. Fatalities have occurred at this beach.
Source: NPS / SF Fire Dept / NOAA — Joint Coastal Safety Advisory (sf-fire.org); NOAA National Weather Service Beach Hazards guidance
Powerful Shore Break
Waves break powerfully onto the beach. The force is sufficient to cause injury and immediately pull people into the water.
Source: California State Parks — Ocean Safety (parks.ca.gov)
No Cell Service
Cell service is limited or unavailable at this beach. Do not rely on your phone to call for help. The trail in is over a mile — tell someone your plans before you leave the trailhead.
Source: California State Parks — Cowell Ranch State Beach / San Mateo Coast (parks.ca.gov)
Cold Water — Year Round
Pacific water here is cold year round. Hypothermia can set in quickly.
Source: California State Parks — Ocean Safety (parks.ca.gov/OceanSafety); NOAA / NPS GGNRA Joint Coastal Safety Advisory
Before You Go Near the Water
01
Read the swell period before you approach the water. The number above is your warning. A period of 15+ seconds means sets are arriving with long quiet intervals between them — sometimes 5 to 10 minutes of calm.
02
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.
03
Wet rocks mark where the ocean has already been. If the rocks are wet, a wave reached them recently. The next set is coming to the same place. Stay above the wet line — always.
04
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.
05
Keep dogs on leash. Keep dogs on leash and away from the water's edge.
06
Tell someone your plans before you hike in. Cell service is limited or unavailable at this beach. Do not rely on your phone to call for help. Leave your destination and expected return time with someone before you leave the trailhead.
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. During the quiet periods between sets, energy is building offshore. People who walk to the water's edge during the lull are swept in when a set arrives.
1
Find a high vantage point above the water line. Do not approach the shore first. Watch from above.
2
Time the quiet intervals. Count the seconds — or minutes — between large waves reaching the shore. This is your set interval.
3
Watch for at least 15 minutes. You need to see multiple sets before you understand the rhythm. One quiet stretch means nothing.
4
The largest wave in each set is not the last one. Sets often build in size. The final wave of a set is frequently the biggest.
5
When in doubt, stay back. If the swell period is above 12 seconds, assume the next set will reach further than the last. At 15+ seconds, stay completely clear of the shore.
6
Long-period swells create “sneaker waves,” sometimes called “sleeper waves,” because they produce sets of waves separated by long periods of relative calm. That calm can create a false sense of security — people approach the shore and are not anticipating the next set when it arrives, leaving no time to retreat. These circumstances are predictable and deaths from being swept in are preventable.
Ocean Emergency
911CA State Parks Emergency · (800) 952-5580
USCG San Francisco · (415) 399-3547