NASA cleared space shuttle Endeavour for a Friday-night launch to the international space station, but the weather outlook was not too promising.
A cold front was making its way toward Florida on Wednesday and was expected to bring rain and thick clouds by the weekend.
Forecasters put the odds of acceptable conditions at 60 percent for the scheduled 7:55 p.m. Friday liftoff and only 40 percent for Saturday.
The chance of success goes up dramatically Sunday.
"The timing of the front is really going to be the major factor," said shuttle weather officer Kathy Winters.
The countdown, at least, was proceeding smoothly, and mission managers gave a thumbs-up at the traditional two-days-before-launch review.
"Everybody is 'go' to proceed on toward launch on Friday," said LeRoy Cain, chairman of the mission management team.
Endeavour and its seven-person crew are set to deliver home improvement equipment that is essential if NASA is to double the size of the space-station crew.
The shuttle is loaded with an extra bathroom and kitchenette, two additional bedrooms, another exercise machine and a recycling system that will turn urine into drinking water at the orbiting outpost.
The plan is to add three more astronauts to the space station by the middle of next year, making for a permanent crew of six.
"Then things are really going to start to get hopping, as they say," the space station's commander, Michael Fincke, said from orbit Wednesday.
Fincke said he and his two crewmates were eagerly awaiting Endeavour's arrival. It's the ride home for Gregory Chamitoff, who has been in orbit since the end of May. He will swap places with one of the astronauts flying up on the space shuttle.
NASA needs to launch Endeavour by Nov. 25; otherwise the flight will be put off until mid-January. That's because of unfavorable sun angles for the docked shuttle and station, and end-of-year shuttle computer concerns.