Wahl's success attracted the attention of the Calgary Flames, who selected him with their second pick, 48th overall, at the 2008 NHL Entry Draft. He also joined the United States men's national junior ice hockey team for the 2009 World Junior Ice Hockey Championships, scoring four points for the fifth place Americans. He finished the 2008–09 WHL season with 32 goals and 63 points. In 2009–10, he finished sixth in league scoring with 96 points and was named a First Team All-Star by the WHL. He ended his WHL career in sixth place all-time in scoring for the Chiefs with 284 points and third all-time in assists with 186. The team also named him one of the top 25 players in their first 25 seasons. Upon the conclusion of his WHL season, Wahl joined the Flames's AHL affiliate, the Abbotsford Heat. He played in four regular season games, scoring a goal and three assists, and remained with the team into the 2010 Calder Cup playoffs.