Shuttle Endeavour is ready for liftoff for its latest mission to space on Saturday; however, forecasters are predicting a 60 percent chance of afternoon and evening thunderstorms and electrical activity that could cause another delay.
Overall, our primary concern on launch day is for thunderstorms and showers to be in the area, particularly within 20 nautical miles of the shuttle landing facility and 10 nautical miles of the launch pad, said shuttle weather officer Kathy Winters.
Good weather is expected at NASA's emergency runways in the United States and Europe, Winters said, so the primary concern will be here at the Kennedy Space Center.
Two previous launch attempts in June were postponed following a potentially hazardous leak in a hydrogen vent line.
The scheduled lift-off is timed for 19:39 EST (23:39 GMT) - if the weather does not intervene.
The shuttle and its crew are set to deliver and install the third and final piece of Japan's $1 billion space station lab, named Kibo, which is Japanese for hope.
If the flight gets away, Endeavour will carry into space a seven-strong crew, including six Americans and one Canadian.
The seven shuttle astronauts plus six station residents will make for the biggest crowd ever in orbit
Endeavour is making the 127th space shuttle flight, and the 29th to the station. Five spacewalks are planned during the 16-day flight.
Seven further flights to the station are planned before the shuttles retire in 2010.