"They'll be watching basically the speed with which this frontal system moves through the Florida peninsula area," said NASA flight commentator Kyle Herring.

Relatively stiff gusts peaking at 18 knots are expected to be blowing down the north end of the runway, so mission managers also will be keeping an eye on wind direction.

NASA flight rules prohibit a landing attempt if crosswinds at the runway top 15 knots, but headwinds up to 25 knots are acceptable.

"The only concern really is whether that wind direction will remain what it is forecast to be - out of the northeast," Herring said. "There is some slight potential that the wind direction could shift around more easterly. That may create a little bit more of a situation with the crosswinds on the runway."

Clouds lingering in the wake of the front also could be a potential problem. Ceilings below 8,000 feet (2,427 meters) would force the Atlantis crew to remain aloft for at least an extra spin around the planet.

The four-man, one-woman crew will have a second opportunity to land at KSC at 2:28 p.m. EST (19:28 GMT).

The conditions at a back-up landing site at Edwards Air Force Base in California are expected to be acceptable for landing, but NASA isn't likely to detour the Atlantis crew unless serious systems problems crop up aboard the shuttle.

The reason: It costs about $1 million to ferry a shuttle back to Florida from the Mojave Desert military base, so NASA officials prefer to land at KSC if at all possible. Consequently, the Atlantis crew likely would remain in space an extra day if conditions prohibited a landing at KSC Sunday.

The weather at KSC is expected to improve by Monday while conditions at Edwards are expected to deteriorate that day. The shuttle has enough fuel and supplies to remain in space until Wednesday.

Launched Feb. 7, the Atlantis crew delivered Destiny to the station two days later. The 16-ton research facility -- which is the first of six science labs that eventually will be launched to the station -- was mounted to the outpost Feb. 9.

Three spacewalks also were carried out to wire up the exterior of the lab so that crucial systems inside the science center could be activated. The shuttle crew departed the station Friday after a weeklong visit to the outpost, now occupied by U.S. astronaut Bill Shepherd and two Russian cosmonauts - Yuri Gidzenko and Sergei Krikalev.

The Atlantis astronauts packed up the shuttle Saturday and successfully tested jet thrusters and flight control systems that play key roles in a return to Earth.

The ship's clamshell-like cargo bay doors are to be closed about 9 a.m. EST (14:00 GMT) Sunday and twin maneuvering engines are to be fired about 11:45 a.m. EST (16:45 GMT), propelling Atlantis and its crew on an hour-long freefall back to the planet.

The landing will close out the first of six shuttle missions to the station in 2001.

Next up: the planned March 8 launch of shuttle Discovery on a mission to ferry a fresh crew up to the outpost.

Russian cosmonaut Yuri Usachev and two U.S. astronauts -- Susan Helms and James Voss -- will make up the station's second resident crew. Shepherd and his two colleagues will return to Earth aboard Discovery on March 20.