The countdown to a launch of space shuttle Discovery moved smoothly Thursday with expected storm clouds remaining the only obstacle to only the second U.S.-operated spaceflight since the Columbia tragedy.

The countdown to a Saturday afternoon launch for a 12-day trip to the international space station was proceeding without any technical problems, said Pete Nicolenko, NASA test director.

"At this point, we're right where we want to be, which is on schedule and tracking no significant issues," Nicolenko said.

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Weather was a different story.

There was a 60 percent chance that thunderstorm clouds bringing lightning and isolated showers would prevent a launch Saturday, and the forecast didn't improve on Sunday or Monday, said Kathy Winters, shuttle weather officer.

The launch time, set for 3:49 p.m. EDT on Saturday, will be bumped up by 22 minutes with each passing day, improving the chances of avoiding Florida's volatile afternoon showers.

"Our main concern will be weather," Winters said. "That launch window moving up earlier is something that might help us out."

Nicolenko said the space agency most likely would try to launch Discovery on Saturday or Sunday, take a break on Monday and then try again on Tuesday.

Saturday's launch would be just the second shuttle flight since the Columbia disaster in 2003, which killed seven astronauts, and the first since the liftoff of Discovery last July.

Discovery's seven-member crew will test shuttle inspection and repair techniques, bring supplies and equipment to the international space station and deliver the European Space Agency's Thomas Reiter for a six-month stay aboard the orbiting outpost.

Astronauts Piers Sellers and Mike Fossum will make two spacewalks and possibly a third, which would add a day to the mission.