The time has come. After two months of waiting for perfect conditions, surfers from across the globe are grabbing their “guns” (specialized big-wave surfboards) and making their way to Northern California to join the Titans of Mavericks big-wave surf competition this Friday. They'll be pummeled by waves the size of four-story buildings, and they'll have Mark Sponsler, the competition's official surf forecaster, to thank.

Mavericks is a notorious break that’s home to some of the heaviest surfable waves in the world. And Sponsler, the founder of Stormsurf.com, is responsible for green-lighting the competition held just north of Half Moon Bay. But the contest doesn’t happen every year; waves must be at least 40 feet high and somewhat structured—as in, not exploding with Poseidon’s rage. Hard to predict in a winter that could produce one of the strongest El Niños on record. That’s where Sponsler comes in.

Most of Sponsler's routine forecasts rely on wave models parsed from National Oceanic Atmospheric Administration weather data, but for important events like Titans, he crunches the numbers by hand. He uses a mix of swell-decay tables—old-school charts that estimate the rate at which swells steadily lose power as they travel through the ocean—and his “secret sauce” of algebraic equations. Then he compares his models against readings from the Jason-2 satellite, which measures sea height to within about an inch. Even so, his models can be thrown off by opposing winds, currents, and other swells.

Sponsler says he’s had his eye on this particular storm for about a week as it has developed over the international dateline. “It was 50-foot seas aimed right at Mavericks."

The storm, which originated roughly midway between here and Japan, some 2,300 nautical miles away, is less preferable than a storm that originates in the Gulf of Alaska, a mere 1,600 nautical miles away, says Sponsler. That's because of that swell-decay rate thing; these waves have had more time to run out of energy than waves that come down from Alaska. It was expected to arrive late Thursday and peak Friday morning. Surfers should see waves between 34 and 36 feet tall (not quite 40, but close enough) with a swell period of ten feet every 17 to 18 seconds. (A swell is essentially a sine wave through the ocean composed of wave after wave.)

He says storms typically follow the jet stream and work their way across the North Pacific. “Mavericks is at the end of that pipeline; all the energy is focused right there, in a perfect place to catch the ball when it comes,” he says.

Sponsler began forecasting in his native Florida, drawing “rustic and inaccurate” wave models based on satellite photos from his local newspaper. Then, when he was working as a software engineer for NASA’s shuttle program in the ’80s, a colleague changed his forecasting forever. “He said, ‘Come here, I want to show you something,’” Sponsler says. “It was the Internet. I was like, ‘I want this.’”

With access to weather data from the entire planet, Sponsler left Florida to hunt waves at Waimea Bay and Sunset Beach in Hawaii—two world-renowned big-wave breaks. Then he heard about Mark Foo’s death at Mavericks. Foo, a legendary Hawaiian big-wave surfer and Sponsler’s acquaintance, drowned after wiping out on a relatively unremarkable 18-foot wave. Sponsler says he thought to himself, “This Mavericks place must be pretty serious.”

Sponsler paddled out for the first time in 1995, with pioneer Jeff Clark, who showed him the ropes. Twenty years later, Sponsler says he has it dialed in. “The game is to paddle out right before the sweet spot, sit there, surf it, taste it like fine wine,” he says. “You learn to pick out the very best barrel in a batch of a whole year’s harvest.” (He draws the line at ginormous waves though. On contest morning, he’ll paddle out with all the competitors and watch just off to the side—the 58-year-old says he’s too old and feeble to “keep up with those madmen.”)

Friday, however, isn't the best storm to have occurred in the past few weeks. The first choice coincided with the Super Bowl, which was a de facto blackout date.

“I can’t believe a bunch of football players trumped a bunch of surfers,” says Sponsler. Now it's the wave riders' chance to get pummeled by enormous forces of nature.

This post has been updated to reflect the start of the event.