Massive, dangerous waves are hitting California. Here’s why

The first huge swell of the winter has arrived in California, and with it comes dangerous surf set to rival some of the biggest waves this year. 

Waves up to 40 feet are possible Thursday and high surf warnings and advisories are in effect up and down the California coast. It’s the first high surf warning issued by the Bay Area National Weather Service office since January, when big waves caused serious damage along Santa Cruz’s iconic West Cliff Drive that’s still being repaired.

Huge low-pressure systems in the Pacific Ocean are pushing large groups of waves toward the California coast.

Three factors determine wave height: wind speed, fetch and time. Fetch is the area over the ocean where winds blow and generate waves, according to the weather service. How much time a weather-making system sits over the Pacific is also crucial, as the longer a storm gathers energy, the bigger the waves it can send to the California coast.

This week, multiple storms have churned over the Pacific Ocean for several days, with broad swaths of hurricane-force winds creating a huge swell. On Wednesday, a 42-foot wave was observed more than 1,000 miles off the California coast.

“As those (swells) come to the coast and they break, that’s when you get those large waves at the coastlines and at the beaches,” said weather service meteorologist Alexis Clouser.

While the California coastline is frequently gusty in the summertime, winds typically only ramp up during the afternoon and evening on a local scale, not a long enough time or large enough area to generate the massive waves of winter storms.

December typically delivers large surf to the California coast. However, Thursday’s waves are expected to be particularly large, prompting the weather warnings. Along the Bay Area coastline, breaking waves are expected to be 28 to 33 feet, and up to 40 feet in places like Maverick’s and 17-Mile Drive.

Waves of 15 to 20 feet are forecast at Point Conception and up to 15 feet in San Diego.

“We really want to hammer home that beaches will be very dangerous,” said weather service meteorologist Nicole Sarment. “People should really not even go.”

The large waves will add up to a foot of storm surge to the coast on top of the annual high tides around the winter solstice. Due to these factors, minor coastal flooding is possible Thursday.

The Bay Area weather service office issues typically only one or two high surf warnings per year. Last winter, a series of storms prompted the office to issue three separate warnings between Jan. 4 and 13.

While the current high surf warning is set to expire Friday morning, large waves will continue through Saturday.

“Just because the warning ends, that does not necessarily mean it’s safe to go into the water,” Clouser said. “We will continue to see these larger waves throughout the next few days.”