NASA says an approaching cold front could bring rain and thick clouds to the launch site by week's end.
Shuttle weather officer Kathy Winters says there's still a 60 percent chance of acceptable conditions at the 7:55 p.m. liftoff time Friday and only a 40 percent chance on Saturday. She says timing is everything in terms of when the weather moves through.
Endeavour is loaded with equipment to expand the living quarters of the space station. NASA hopes to double the size of the crew, from three to six, by next June. The launch window is open until Nov. 25 for NASA's first shuttle launch since the end of May.