High crosswinds at the Kennedy Space Center blocked two back-to-back landing attempts by shuttle Atlantis today and forced entry flight director Leroy Cain to extend the mission by one day in hopes of better weather Monday.

"We've been watching the winds really closely and the peaks have stayed at or just beyond the (safety) limit," astronaut Scott Altman radioed the crew from Houston at 12:47 p.m.

"It just does not look like an optimum day to come home," he continued, "so we are going to wave off and go to a 24-hour extension."

"OK, Scooter, we understand," replied commander Kenneth Cockrell from Atlantis.

Deorbit ignition is now targeted for orbit 185 at 12:20:46 p.m. EST (1721 GMT) for a landing on runway 33 at 1:26:55 p.m. (1827 GMT). A second Florida landing opportunity is available one orbit later at 3:03:21 p.m. (2003 GMT).

Cockrell and company originally planned to land at 12:53 p.m. today. But just before 11:30 a.m., as timers were counting down to deorbit ignition, Cain ordered a one-orbit wave off because of crosswinds that were gusting to more than 18 knots.

The shuttle makes an unpowered, gliding approach to the landing site and NASA's daytime end-of-mission crosswind limit is 15 knots.

As Atlantis made another lap around the planet, the winds shifted direction slightly, but not enough to reduce crosswinds below the safety limit. In fact, Altman radioed, they appeared to be increasing slightly.

With forecasters predicting the high winds probably would continue, Cain ordered a 24-hour wave-off and told the crew they would try again Monday.

"We appreciate your patience with us while we were putting this story together," Altman told the astronauts. "We just think tomorrow's forecast looks like a lot better chance to get you safely into KSC."

Cockrell, pilot Mark Polansky, flight engineer Marsha Ivins and spacewalkers Thomas Jones and Robert Curbeam immediately began backing out of the deorbit timeline, opening the shuttle's cargo bay doors and setting up shop for extended orbital operations.

The astronauts will have two landing opportunities in Florida Monday and two at Edwards Air Force Base in California's Mojave Desert.

The forecast calls for generally good weather at Kennedy with crosswinds well within limits, but there is a chance a broken deck of clouds could develop at 4,000 feet. NASA flight rules forbid a landing if clouds are forecast to go broken below 8,000 feet.

The weather at Edwards, meanwhile, appears solidly "no go," with two decks of broken clouds at or below 8,000 feet, winds from 220 degrees gusting to 22 knots and a chance of showers in the vicinity.

If that forecast holds up, NASA might elect to forego staffing Edwards on Monday and to concentrate solely on getting Atlantis back to the Kennedy Space Center.

During a status briefing Saturday, Cain said Atlantis had enough propellant, electrical power and crew supplies to remain in orbit until Wednesday in a worst-case scenario.

Conditions at Kennedy, however, are expected to improve throughout the week and flight controllers are optimistic about getting Atlantis back on the ground in Florida Monday or, at worst, Tuesday.