Melanie Segard, 21, will begin training for the broadcast next Tuesday at the studios of public television station France 2, UNAPEI said in a statement.
Segard has scored more than 200,000 "likes" on Facebook in less than two weeks since the association launched a public awareness campaign for Down's Syndrome titled "Melanie Can Do It".
"Hi everyone, my name is Melanie and my dream is to present the weather," she says in the post.
Down's Syndrome is a genetic condition with no known cure that typically affects a person's physical and intellectual growth.
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It affects approximately one in 1,000 people, according to the World Health Organization.
Also known as trisomy 21, the condition is caused by the presence of an extra, or third, copy of chromosome number 21.
"Melanie is our ambassador," UNAPEI president Luc Gateau told French radio. "We have just proved that a person with a handicap can realise her dream," he said, adding: "Society needs this.