Going to the beach this Fourth of July? Here’s what you can expect in SLO County

This Fourth of July holiday weekend will see beach closures in Northern and Southern California and triple-digit temperatures throughout California’s inland areas that will drive many to the Central Coast for relief from the hot weather.

They will be met with the triple threat of shrinking beaches, rogue waves and large crowds, increasing the threat of COVID-19 exposure.

When they arrive along the shoreline, beachgoers will see two wave trains converging: an 8- to 10-feet northwesterly sea and swell, and a long-period and powerful 3 to 5-foot Southern Hemisphere swell (with a 17- to 19-second period) that will create a turbulent ocean.

To make matters worse, high surf will coincide with some of the highest astronomical tides of 2020. A 6.6-foot high tide will occur about an hour after sunset, a time when many people light personal fireworks on the beach.

This will be especially problematic, because a flood high tide will crowd more people onto a shrinking beach as the level of the sea rises that evening. This could increase the threat of transmission of coronavirus.

But there’s another danger: Sneaker or rogue waves can drag you into the frigid sea, and here is why.

What are rogue or sneaker waves?

Rogue waves occur in several ways. One of the most common causes is when wind waves with different periods or wavelengths meet in a single spot and complement each other as part of a general process called interference.

Destructive interference occurs when the different wave trains are 180 degrees out of phase and cancel each other out.

Constructive interference occurs when the different wave trains are in phase, and two smaller waves coalesce to produce a massive wave for a short time. This additive formation of the large crest and deep troughs can suddenly cause waves to develop with enormous size.

In oceanography, rogue waves are defined as waves whose height is more than twice the significant wave height. Significant wave height is defined as the average height of the waves in the top third of the wave record. That turns out to be very close to what an experienced mariner — an “old salt” — would perceive the wave heights.

These Southern Hemisphere swell trains are notorious for having extended periods between wave sets, which can lull beachgoers into a false sense of security, before the so-called “sneaker waves” rear their ugly heads.

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.

Over the last few weeks, the ocean has been relatively warm, with ocean temperatures hovering around 60 degrees Fahrenheit.

However, this weekend’s moderate gale-force to fresh gale-force (32 to 46 mph) northwesterly winds along the coastline will produce vast amounts of upwelling. And seawater temperatures will drop to 50 and 54 degrees, increasing the risk of hypothermia if you are dragged into the unsympathetic and unforgiving ocean.

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. Three people were recently killed in southern California after being swept off rocks by a wave.

So, stay physically distant from others, wear a mask, keep your eyes on the ocean and stay situationally aware of your surroundings.