McJames led after the first run by just .03 over Peterson.
"Today I wanted it, and it was tough conditions, but I think if there's anything I've learned in the last four years of being an independent skier it's that perseverance is the name of the game," McJames said. "I've been skiing well and I'm proud of it, and so I'm happy that it paid off today."
Gusting winds and low visibility were factors in only 36 of 66 skiers finishing their opening run. The starting gate for the second run was moved down several gates on the Narrow Gauge course.
"I surprised myself with my results this year just given I am a full-time student and I'm not training full time," Peterson said. "It definitely feels pretty sweet. It was some of the craziest conditions I think I've ever raced in, but I was so confident in the snow because it was so good first run so I just knew that I could stand on it and push it."
Super-G champion Laurenne Ross did not finish and slalom winner Resi Stiegler took a fall.