Mission Control told the shuttle crew early Saturday the initial weather forecast for the first opportunity was for light wind and no rain, ideal conditions for Discovery to land.
A slight chance of showers was forecast at the time of the second opportunity.
"Right now it looks pretty good," Mission Control said.
The shuttle crew began the landing process by closing Discovery's payload bay doors, which are kept open during flights to keep heat from building up in the spacecraft.
Since Discovery has enough supplies to allow it to stay in orbit until Tuesday, NASA will only aim on Saturday to bring home the shuttle in Florida, the primary landing site.
Florida would remain the only choice on Sunday as well, if Saturday's opportunities don't pan out, said flight director Richard Jones.
The backup site of Edwards Air Force Base in the California desert would not be considered until Monday.
NASA engineers said a metal clip that broke off Discovery's rudder on Friday poses no threat to the shuttle's landing. The clip broke loose during a routine checkout of the flight systems needed for Saturday's descent.
Space shuttles have returned to Earth in the past with clips missing from the rudder, which opens like a book to serve as an air brake. The V-shaped spring tabs, just 2½ inches by 1 inch, protect that area from the intense heat of liftoff by providing a seamless barrier down the back edge of the rudder, or tail, panels.
Engineers suspect this clip, or tab, came loose during the vibrations of launch on May 31, and it wasn't until the brake panels were opened for checkout Friday that it floated away.
Ever since the Columbia tragedy five years ago, any shuttle part seen floating away in orbit is quickly and intensely studied by NASA.
A protrusion in the same area at the tail, reported by the astronauts around the same time, also was found to be harmless.
Discovery's crew of seven is returning after delivering and installing the new lab named Kibo, Japanese for hope, to the space station.
Besides delivering the lab, the shuttle also dropped off Gregory Chamitoff, the station's newest crew member. He traded places with Garrett Reisman, who lived on the station for three months. Chamitoff will stay for six months.
The 37-foot lab, about the size of a bus, is the biggest room at the space station. Kibo also has a storage closet and a 33-foot robotic arm. A final section - a "porch" for exterior experiments - and a second, smaller robotic arm will be delivered next year.
Discovery also delivered a pump that fixed the space station's malfunctioning toilet.