Could the California Central Coast see 74-foot waves smashing into beaches?

I’ve been getting emails and phone calls asking if we would ever see waves as high as 74 feet — like those seen off the coast of Northern California in late November — smashing into our Central Coast beaches?

We would not, and here’s why.

The longer the period of the swell, the longer its wavelength will be.

The wavelength determines how far down in the water column the wave can be felt. Waves will begin to feel the ocean floor at half their wavelength.

For example, a swell with a 20-second period will start to feel the ocean bottom at about 1,000 feet, while a wave with a 5-second period won’t feel the ocean floor until 80 feet.

The Diablo Canyon Waverider Buoy is moored in about 100 feet of water. If you were scuba diving at its anchor, you wouldn’t feel any surge from locally generated short-period seas overhead.

However, if the period of the swell were 20 seconds, the surge from each passing wave would toss you back and forth along the ocean’s floor.

As waves feel the bottom, they bend toward shallower depths, which changes their incoming direction and other characteristics. That refraction can have profound and unique effects at the various surf breaks along the California coast.

More importantly, when these deep-water waves began to feel the bottom, their wavelengths dramatically shorten and their speed slows down. But their wave periods remain the same.

The base of the wave is decelerated in shallow water by the drag of the seabed. Due to the wave’s shorter wavelengths, as they interact with the ocean’s floor, they quickly reach a 7-to-1 ratio of wavelength to wave height and break in shallower waters closer to the coastline.

These waves will reform with a shorter period, causing the waves to break at lower heights as they approach the shallower waters along the beaches.

At Point Reyes, you can see this cycle occur numerous times before the waves finally reach the exposed beige colored sands, much smaller than they were just a few miles out the sea.

The record-breaking storm that made landfall near the California and Oregon border in late November was remarkable.

This mid-latitude cyclone dropped the atmospheric pressure to 973.4 millibars, or, 28.69 inches of mercury, smashing the previous all-time low-pressure reading of 977 millibars in California.

The southerly winds at the Crescent City Airport reached 56 mph sustained with gusts to 69 mph.

Off Oregon, this 972-millibar storm combined with a 1037-millibar area of high pressure nearby, created an extraordinarily steep pressure gradient that produced sustained southerly winds of 85 mph with gusts to 106 mph at Cape Blanco.

By the way, sustained winds of 74 mph or higher are hurricane force.

The Cape Mendocino buoy (No. 094), run by Scripps’ Coastal Data Information Program (CDIP), is a deep-water marker moored in 1,132 feet of water. That buoy reached a significant wave height of 43 feet with a maximum wave height of 74 feet on Nov. 26.

On the other hand, moored in only 361 feet of ocean depth, the Humboldt Bay Waverider Buoy (No. 168) is a shallow-water marker, much like the Diablo Canyon’s Waverider.

Even though the Humboldt Bay Waverider Buoy was closer to the storm’s wind fetch (the distance of water over which the wind blows) or area of highest wave height, it reached 37 feet with a 15-second period and a maximum wave height of 48 feet. That’s the highest significant wave events recorded at this location.

As these big waves traveled eastward into the shallower water toward the shoreline, they continued to break — significantly reducing their height before reaching an estimated height of between 17 to 23 feet along many of the beaches.

As big as these Thanksgiving holiday waves were, they were bigger back in 2016.

The National Oceanic and Atmospheric Administration (NOAA) Marine Buoy (No. 72), 230 miles southwest of Dutch Harbor and located within the Aleutian Islands of Alaska, reached 56 feet with a 17-second period on Dec.12, 2016, as a 948 millibar storm with hurricane force winds moved through the Bering Sea.

Understandably, the buoy stopped transmitting wave data later that day. The 56-foot reading was “significant wave height,” which is defined as the average height of the waves in the top third of the wave record.

There is a chance, statistically speaking, that one wave in 1,175 could have reach over 100 feet in these types of conditions.

Overall, It takes unique bottom topography on the ocean floor to create monster waves like those that are found at Mavericks, Half Moon Bay, Hawaii’s North Shore and Nazaré, Portugal.

Surfer lore will tell you the highest waves come in the middle of the wave train.

In the middle of the group, the wave crests and troughs are in phase with each other and add together for maximum height. This is the so-called seventh wave.

Remember, if you go the coast to watch these waves, never turn your back on the ocean. Some of these waves can surge more than 150 feet up the beach.

’Twas the Night Before Christmas’

I will narrate “ ’Twas the Night Before Christmas” at the Atascadero Community Band’s Holiday Sampler at the Pavilion on the Lake in Atascadero on Sunday.

The band will perform popular and traditional holiday music, including an old-fashioned sing-along, from 2:30 to 4:30 p.m.

Admission, refreshments and parking are free. This holiday concert will benefit the Toy Bank of Greater Paso Robles, a nonprofit group that helps provide approximately 1,500 children with holiday cheer.