Weather Watch

Seawater here is coolest in spring

Whether you are a surfer, a scuba diver, fisherman or just a casual beach visitor, you probably care about the temperature of the ocean along our coast. So does Tribune reader Nicolas of Los Osos, who asked me at what time of year we have the coldest and warmest seawater temperatures.

I have a lot of experience with that subject. For many years, one of my jobs at the Diablo Canyon nuclear power plant was to monitor and record seawater temperatures along the Pecho Coast as part of the plant’s environmental monitoring program.

This interesting job was accomplished by electronic waterproof seawater temperature recorders, which are currently about the size of a large Sharpie.

These temperature recorders are housed in steel canisters and placed at fixed locations in the intertidal and subtidal zones. Depending on the tidal cycle, intertidal canisters are exposed to the air at minus low tide and underwater at high tide; the subtidal locations are always underwater.

Depending on the weather, waves and tides, the temperature recorders are swapped out every few months.

During the summer, the minus low tides occur during the O-dark hours before dawn. Over the decades, an anthology of marine biologists has journeyed to the tide pools and swapped out these temperature recorders by flashlight or by the light of the waxing gibbous moon due to the luni-tidal interval. If the conditions are right, bioluminescence from plankton blooms and/or the deep scattering layer can mark your track across the cool, wet and slippery rocks of the littoral.

During swap-out, the new recorders are installed in the canisters and the old recorders taken back to the Diablo Canyon Ocean Lab to be downloaded to a computer database, serviced and calibrated.

This process has been going on since 1976, and has produced an amazingly rich temperature database containing millions of recordings.

To answer Nicolas’ question, I decided to use data from the Patton Cove subtidal temperature monitoring station, which is just south of the power plant in approximately 10 feet of water. This station has been in use since 1976 and more than 800,000 temperature data points have been recorded there.

The average yearly seawater temperature at this location is a cool 55.3 degrees Fahrenheit. The average minimum temperature usually occurs during the spring, and reaches a bone-chilling 48.8 degrees. During the fall, the average maximum temperature can rise to 62.5 degrees.

Over the last week, relentless northwesterly winds have finally decreased. This condition has resulted in less upwelling and warmer seawater temperatures along with a stubborn marine layer along the coast.

As we move toward fall, the seawater along our coastline should continue to warm, allowing for more comfortable conditions for the surfers and scuba divers, with the promise of albacore gathering closer to the shoreline.

Today’s forecast

A 1,010-millibar low-pressure system is centered over the northern Bay Area with an associated trough of low pressure extending along the coast. This trough will gradually dissipate through Monday as a 1,024-millibar Eastern Pacific high creeps toward California.

The coastal stratus will burn off from the interior later this morning and from the coastal valleys by this afternoon, leaving behind mostly clear to partly cloudy conditions as mid-level subtropical moisture streams in from the south.

Along the coast, marine low clouds will be stubbornly persistent, with periods of sunshine in the afternoon. Temperatures will range from the low to high 60s. The marine low clouds will move inland again tonight through Monday morning. The coastal stratus will burn off from the interior and coastal valleys later Monday morning, but will be once again slow to clear from the beaches because of the lack of any strong winds.

Starting Tuesday, temperatures will warm up a few degrees, reaching the century mark Wednesday in the interior and the low 80s in the coastal valleys. Increasing northwesterly winds will produce a greater amount of clearing and warmer temperatures along the beaches, especially the southwesterly facing (Avila Beach and Cayucos) beaches, where temperatures may reach the high 70s by weekend.

Surf report

Increasing northwesterly winds off Cape Mendocino will produce a 3- to 4-foot northwesterly (310-degree deep-water) swell (with an 8- to 10-second period) today, increasing to 3 to 5 feet (with an 8- to 14-second period) Monday.

An early season 970-millibar storm that moved northeastward along the Aleutian Islands toward the Gulf of Alaska will produce a 4- to 6-foot northwesterly (290-degree deep-water) swell (with a 12- to 17-second period) Tuesday. This swell will remain at this height but with a gradually shorter period through Wednesday.

Increasing northwesterly winds will generate 3- to 5-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 11-second period) Thursday and Friday.

Arriving from the Southern Hemisphere: This morning’s 1- to 2-foot Southern Hemisphere (220-degree deep-water) swell (with a 17- and 19-second period) will remain at this height but with a gradually shorter period through Tuesday.

Another Southern Hemisphere swell will overlap Tuesday’s swell, producing a 1- to 2-foot (220-degree deep-water) swell (with a 14- to 20-second period) Wednesday and Thursday.

A significant storm is expected to develop about 1,500 miles east of New Zealand on Tuesday. If this storm develops as advertised, a long-period Southern Hemisphere swell should arrive July 20.

Seawater temperatures: The lack of northwesterly winds has diminished upwelling and has also produced a persistent northerly (uphill) flowing current. This condition has produced warmer seawater temperatures.

Seawater temperatures will range between 54 and 56 degrees today, increasing to 55 to 57 degrees Monday and Tuesday.

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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. Send him an e-mail at