A cold front was making its way toward Florida 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.