"The Eddie," as it's known, is only held when waves in Waimea Bay top 20 feet, which hadn't happened since 2009. Waimea only gets huge swells in the winter, so organizers block out a three-month window from December to February and hope for a Pacific storm.
They got their wish this year -- with just days to spare.
A Pacific storm with hurricane-force winds north of Hawaii churned up powerful swells that pounded surfers for hours.
Florence scored 301 points on his four waves, beating Australian Ross Clarke-Jones, who had a near-perfect run in the second round. Hawaiian Shane Dorian came in third.
The memorial competition was held for the first time in 1984 in honor of legendary big-wave surfer Eddie Aikau. He was also the first official lifeguard on Oahu's North Shore, and was lost at sea in 1978, after volunteering to go for help when a canoe trip seeking to recreate an ancient Polynesian migration hit bad weather.
Aikau's younger brother Clyde won the second time the memorial competition was held, in 1986-87.
Clyde Aikau thanked the crowd before the competition and was thrilled with the surf conditions.
"Today has to be one of the best days I've seen in 40 years," he said.
The 66-year-old paddled out later in the day to compete in his final Eddie before retiring.
He ended up finishing 20th.
The huge waves and unpredictable nature of the competition have made it a popular event with surfers and locals.
Some fans camped out on the beach or slept in their cars to get a good spot, and the roads were crowded in the predawn hours.