That said, wave forecasters have discovered that even some lower-power hurricanes, like Sandy, can make very large swells if they develop something called "trapped fetch waves" that last for more than 18 hours.
"When Atlantic hurricanes make the turn and go up the East coast of the US, they travel in a straight line," explained Stripling. On the right side of such storms the winds are blowing north, in the same direction the storm is moving. If the storm's speed syncs with the waves it's generating and they are all traveling in the same direction, the winds just keep growing the same waves bigger and bigger as the storm moves up the coast.
"It was the Canadians who noticed it first, when they were being pounded by incredibly big waves that were way larger than expected," said Stripling.
The final part of the recipe for giant surfing waves is the place where the waves break. The geometry of the sea bottom and coast can focus the waves, making them taller as well as shaping how they break. This is why there are certain places famous for their giant waves – when the swells are there. Places like Nazaré, Mavericks and Cortes banks in California, Jaws in Maui, and many other big wave sites worldwide are watched religiously by surfers and Surfline, so that the world's big wave surfers, like Garret McNamara, can hop on planes and arrive in time to catch the largest waves, which may only last a few hours.