How a local surfer with a tech background created a global go-to for predicting swells

The Mavericks Wave, near Pillar Point Harbor in El Granda. (Courtesy of Powerlines Productions)

By August Howell

This is the time of year when Mark Sponsler’s phone starts ringing and his website numbers begin climbing.

“It’s really interesting. You watch the traffic on the website go from really sleepy during the summer to instantly, beginning around mid-August, start to creep up,” Sponsler said. “It’s pretty high right now because people think, ‘Finally, there’s going to be some surf.’”

Mark Sponsler launched Stormsurf back in 1998. “Mark is really passionate about his work. He spends endless hours pulling information out,” says surf legend Jeff Clark. (Image via Mark Sponsler)

Summer months are usually inconsistent for a surfer in California. It’s the fall and winter seasons that typically provide the most consistent and powerful surf. With summer in the rearview mirror, surfers will begin to check online sources to help predict when and where their next session will take place. Sponsler’s surf and weather forecasting website,, has become the go-to source for surfers looking to test their mettle at Mavericks, the famous big-wave break located off Pillar Point.

Whether users are visiting big-name professional surfers, seasoned locals or even movie producers, Stormsurf provides anyone with even a mild curiosity about local conditions a multitude of options for checking the surf. There are plenty of factors to consider, and big-wave spots like Mavericks, which breaks from large storms in the middle of the Pacific, can be tricky to predict.

The website delves into the minutiae of swells, analyzing wave height, period, direction, wind speed and direction, and tides for different regions across the globe. The information is all free. Sponsler, 63, launched Stormsurf in 1998 intending to focus the computer power on predicting when massive waves would break at Mavericks.

“If you don’t go out and sample the fruits of your labor, then you’ll never know how right or wrong you are.”—Sponsler surfing Mavericks. (Courtesy of Powerlines Productions)

He might be one of the few surf forecasters around to check his work firsthand when it comes to dangerous surf. Sponsler, who lives in the East Bay, is an avid surfer on the San Mateo County coast and was a common face in the Mavericks lineup for years. It was in that lineup that he met Mavericks pioneer Jeff Clark. The two worked together to study data after each swell, building the site’s reputation and accuracy. Clark believes the site has remained popular because Sponsler is devoted to his forecasting and puts the time in to get it right.

“Mark is really passionate about his work,” Clark said. “He spends endless hours pulling information out.”

What began as an email thread between local surfers in 1995 turned into a sophisticated forecasting site. Today, Stormsurf forecasts for more than 50 regions worldwide, from Indonesia to New York to South Africa. It’s the result of trial and error and a lot of computer processing. Sponsler creates his own models from downloadable data sets provided by the National Oceanic and Atmospheric Administration.

“There was data, but there weren’t mountains of data,” Sponsler said of the site’s early days. “There weren’t multiple models you could compare and contrast.”

Because of the consistency of his predictions, Stormsurf played a key role in every contest held at Mavericks. He also had a role in the 2012 film “Chasing Mavericks,” notifying producers when the best weather and swell window would be for shooting. When Clark was the contest director, Sponsler’s data was one of the factors in determining when the Mavericks contest would get a green light.

Sponsler comes from a tech background, having worked for Lockheed Martin at the Kennedy Space Center. Implementing better technology on the site was only one part of the equation, according to Sponsler. To become a good — and accurate — forecaster, he had to put in the time in the water to get a true sense of how the elements come together. That meant going out to Mavericks each swell and comparing the buoys and weather charts.

Sponsler grew up surfing small waves in Florida. The hurricanes there sparked his interest in meteorology. When he moved to the Bay Area in the 1990s, he had to learn more nuances of reading fast-moving, long-period swells on the buoys. As he put it, “It’s just experience of correlating the maps to reality.

“If you don’t go out and sample the fruits of your labor, then you’ll never know how right or wrong you are,” he said.

(Editor’s Note: This article originally appeared in the Half Moon Bay Review. This version is a cross-publishing collaboration between our two publications.)

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