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Coming into the 10-man final having qualified second in his heats, Porteous went straight to the top of the leaderboard with a score of 93.75 on his first run of three.
Double Olympic gold medallist David Wise (US) laid down a strong challenge but his top score of 90.50 put him just behind Porteous.
Top qualifier Aaron Blunck (US) recovered from a first-run crash to leapfrog into first place with a score of 96.25 on his second lap through the 6.7m-high, 20.7m-wide superpipe.
Knowing that he still had the opportunity to lift his score, Porteous attempted to add another double at the end of his third run but could not quite pull it off. He was happy enough to settle for a silver medal at the season-opener.
"I’m pretty over the moon with how today went," he said.
"I had a really great training and then managed to put my first run down with a 93 which I was really stoked to be able to do. I can’t wait to add a few more tricks in there and I’m excited for Dew Tour next week."
New Zealand’s Olympic bronze medallist, and youngest of the Porteous brothers, Nico, finished in fourth place with a top score of 88.00 on his first run.
He attempted a 1440 on his second run but missed the grab and had to settle for a 1260, and no improvement in his score.
Beau-James Wells crashed in practice and was unable to start the finals. He was to have a medical assessment.