Atlantis orbits the Earth on Saturday evening. Photo: NASA TV/Spaceflight Now

Rainshowers and gusty crosswinds blowing across Kennedy Space Center's swamp-surrounded runway may thwart NASA's plans to bring shuttle Atlantis back to Earth in the wee hours on Monday morning.

Commander Jim Halsell is scheduled to fly Atlantis to a 220-mph touchdown in darkness at 2:20 a.m. EDT (0620 GMT), but a cold front sweeping through Florida might force the shuttle to remain in space until Tuesday or Wednesday.

Follow the landing with our live play-by-play reports in the Mission Status Center.

Entry Flight Director John Shannon described the weather picture to reporters early today: "There is a front tied to a low-pressure system that will be off the East Coast. That front extends down through Georgia and the Florida panhandle. It really depends how close it gets to the Cape. It will pushing rain storms in front of it and higher winds."

Rigid NASA rules say the shuttle cannot land if there is rain with 30 nautical miles of the runway because flying through precipitation could damage the spaceplane's delicate thermal-protection tiles.

In addition, crosswinds must not exceed 12 knots for a nighttime landing. The forecast for Monday is calling for crosswinds gusting right at the limit.

If weather does not permit an on-time homecoming Monday, the space agency will first send Atlantis around the world one additional time in hopes conditions in Florida improve an orbit later.

The second and last chance at a KSC landing Monday would occur at 3:56 a.m. EDT (0756 GMT).

However, conditions probably won't be any better 90 minutes later, Shannon said.

"The general pattern is the first opportunity is going to be the best. We are going to be trying to beat that front and all the rain clouds that will be pushed ahead it."

Illustrations show Atlantis' track to Kennedy Space Center during Monday's two possible landing opportunities. Photos: NASA TV/Spaceflight Now

The winds are forecast to remain stable through the night, not changing significantly from one landing opportunity to the other.

Chief NASA astronaut Charlie Precourt will be piloting the Shuttle Training Aircraft for weather reconnaissance at the landing site.

The modified Gulfstream jet mimics the flying characteristics of the shuttle and Precourt's approaches to the runway will give Mission Control officials a true insight into the conditions Atlantis' crew would face.

"He is able to give us very valuable weather recon information on the handling qualities of the STA," Shannon said. "We look to Col. Precourt to tell us things like the bank angle, the turbulence, things we could not get from a weather balloon or from a wind tower."

Should the weather remain unacceptable, NASA plans to keep Atlantis aloft until Tuesday, or even Wednesday if necessary. Senior officials decided on Saturday to forgo using the backup landing site at Edwards Air Force Base in California until Wednesday morning at the earliest.

NASA wants to avoid landing on the West Coast because of the week it would take to ferry Atlantis back to Florida, delaying work to ready the ship for its next space flight in early September. The cross-country haul also costs the space agency around $1 million.

The early weather forecast for Tuesday in Florida appears even worse than Monday.

"The meteorological models show that the front will be just past Kennedy Space Center for the Tuesday. You would have a greater potential for rain if that were true. But those models have some error...and it is too early to call," Shannon explained.

Tuesday's two landing opportunities are 1:16 a.m. EDT (0516 GMT) and 2:51 a.m. EDT (0651 a.m. GMT).

Atlantis has enough fuel, food and other consumables to safely remain in space until Thursday.

Previewing landing day
The seven shuttle astronauts will be awakened at 4:11 p.m. EDT (2011 GMT) Sunday afternoon. Through the final hours in space, the crew will finish putting away equipment while transforming Atlantis from a spacecraft back to a flying machine for the journey home.

The 60-foot long payload bay doors will be closed and locked at 10:33 p.m. EDT (0233 GMT on Monday), followed by transitioning the shuttle's onboard computers from the OPS-2 on-orbit software to the OPS-3 package used for landing.

After donning their day-glo orange launch and entry spacesuits, the astronauts will start drinking large amounts of fluids to aid in readapting to Earth's gravity upon landing.

The astronauts conducted a test of Atlantis' aerosurfaces on Saturday evening in preparation for Monday's landing. Photo: NASA TV/Spaceflight Now

If the weather cooperates, the astronauts will fire the shuttle's twin orbital maneuvering system engines for three minutes and 7 seconds beginning at 1:12:35 a.m. EDT (0512:35 GMT). The burn will slow Atlantis by about 313 feet per second, enough to drop the 110-ton shuttle from orbit to start the hour-long dive through Earth's atmosphere.

Atlantis should begin feeling the upper fringes of the atmosphere at about 1:48:28 a.m. EDT (0548:28 GMT) at an altitude of 398,500 feet, some 4,406 nautical miles from KSC at a point known as Entry Interface.

At 2:16:43 a.m. EDT (0616:43 GMT), commander Halsell will make a 243-degree left-overhead turn to align Atlantis with Kennedy Space Center's Runway 15. Touchdown is expected at 2:20:11 a.m. EDT (0620:11 GMT).

Within the hour-long trek home, Atlantis will have traveled from the southern Pacific Ocean, heading northeastward to KSC, with landfall over southern Mexico, then passing across the Gulf of Mexico before entering the state of Florida near Fort Myers.