As Atlantis soared overhead, right around what would have been landing time, commander Scott Altman called down that he saw a "pretty solid mass of clouds".
"It looks to us like maybe it is starting to break up and move out, so we're hopeful to get home tomorrow," he said.
"We're sharing your optimism," Mission Control replied.
Despite an equally dismal forecast for Sunday, Mission Control opted to wait out the bad weather rather than move the landing to California.
Next Nasa chief
Nasa mission control chiefs will re-examine prospects for a safe landing on Sunday.
As the astronauts settled in for another day in space, Barack Obama, the US president, announced that Charles Bolden, a former shuttle commander, was his choice for Nasa's next administrator.
Obama told the Atlantis crew earlier in the week by telephone that an announcement was imminent.
Mission Control relayed the news to the astronauts two hours later. "I guess it wasn't any of us in the flight after all," Altman joked.
If confirmed by the senate, Bolden would become the space agency's first black administrator and only the second astronaut ever to hold the post.