Champion will anchor the network's new flagship morning show, which is set to debut in early 2014, The Weather Channel president David Clark said Monday.
Based at The Weather Channel headquarters in Atlanta, Champion will also assume managing editor responsibilities at the network.
"With his commitment to understanding, predicting and explaining the power of weather in our lives, Sam perfectly embodies The Weather Channel brand's philosophy, 'It's Amazing Out There,'" Clark said.
Champion called his new post "a once-in-a-lifetime opportunity for me to do what I love most at a network that lives and breathes the weather."
His last day on "GMA" is Wednesday. He had been on that program since 2006, and joined ABC-owned local New York station WABC in 1988.
"For 25 years, Sam has been a vital member of the ABC News family," said ABC News president Ben Sherwood in announcing Champion's departure.
He said Champion's fellow meteorologist Ginger Zee will take over his weather responsibilities at "GMA" and across the news division. Zee became weather anchor for the weekend edition of "GMA" two years ago.
Champion has been a member of the "GMA" anchor team as it seized the ratings crown from NBC's perennially front-running "Today" show. Whether his absence from the "GMA" anchor desk affects its ratings dominance will be closely tracked by morning-TV handicappers.
And he leaves the network scrambling to keep intact the rest of its winning "GMA" ensemble, led by anchor George Stephanopoulos. Josh Elliott, Robin Roberts and Lara Spencer are reportedly negotiating new contracts.
Ironically, in his new role, Champion now joins forces with his former rival, "Today" weather co-host Al Roker, whose one-hour "Wake Up With Al" airs on The Weather Channel weekdays at 6 a.m. EST, then repeats at 10 a.m. Champion's new morning show will be tucked in between, airing from 7 to 10 a.m., the network said.
Meanwhile, Champion will also be in the partial employ (in a sense) of his old network rival: The Weather Channel is owned by a consortium made up in part of NBC Universal.
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