Stormy weather could postpone the last-ever flight of the US shuttle program by delaying Atlantis's final mission to the International Space Station a day or two, NASA said Wednesday.

A tropical storm system moving north from the Caribbean is expected to bring rain, clouds and thunder to Florida right around the time of Atlantis's scheduled liftoff Friday at 11:26 am (1526 GMT).

"It is not looking favorable right now for launch," said NASA weather officer Kathy Winters, downgrading her forecast to just a 30 percent chance of good conditions for liftoff, compared to 40 percent a day earlier.

Other than the weather, there are no technical issues that would prevent Atlantis from embarking on its 12-day journey to the International Space Station, marking the final flight of the 30-year US space shuttle program.

"The countdown so far is going extremely well," said shuttle manager Mike Leinbach.

Other launch opportunities open on Saturday and Sunday when the weather forecasts are clearer but still far from perfect in the southern United States where summer storms are frequent.

"Each day gets a little bit better," said Winters, noting that by Sunday the weather forecast improves to a 60-percent likelihood of favorable conditions for launch.

Nevertheless, the mission management team met early Wednesday and decided unanimously to go ahead with preparations for launch, NASA said.

"There are no problems yet, there are just weather forecasts," said shuttle launch integration manager Mike Moses.

If the shuttle does not take off on Friday, NASA will likely rule out the option of adding an extra day to the mission, which is mainly aimed at toting supplies to re-stock the International Space Station.

Four US astronauts will be on board, a skeleton crew compared to the six or seven who normally ride a shuttle to space.

The mission team is not disheartened by the forecasts of damp weather, and is proceeding as if the launch will go ahead as planned, said Leinbach.

"Friday is game day for us. We don't want to wait until Saturday. We want to play the game Friday," he told reporters.

NASA engineers will meet again to examine the latest weather forecasts on Thursday before the rotating service structure is pushed away from the shuttle, in the final major movement on the launch pad before the spacecraft launches.

If weather looks bad, they may decide to postpone the rollback. Otherwise, the next major decision-making moment comes before the external fuel tank is filled early Friday morning, around 1:30 am (0530 GMT).

"Before we go load the propellants into the tank we'll take a look at the weather and make sure it's really a good day to try that, and so at that point we'll be making a decision," said Moses.

The weather forecast for tanking time time is just 20-percent favorable, Winters said.

When Atlantis returns from its final journey, the shuttle will become a tourist piece at Kennedy Space Center, while its sister shuttles -- Enterprise, Discovery and Endeavour -- go to other museums across the country.

The end of the US shuttle program means the world's astronauts will have to rely on Russia's three-seat space capsules for transit to and from the International Space Station at a cost of $51 million per seat.

Private companies are currently competing to build a next-generation US spacecraft that can transport people to the orbiting research lab, but that vehicle is not likely to be ready before 2015 at the earliest.

In Washington, President Barack Obama said he was "proud" of the shuttle's legacy but that NASA needs to make new leaps forward so it can explore Mars and an asteroid in the future.

"Let's ultimately get to Mars. A good pit stop is an asteroid," said Obama, who shortly after taking office axed NASA's Constellation program that would have returned astronauts to the moon.

"Let's start stretching the boundaries so we're not doing the same things over and over again. But rather, let's start thinking about, what's the next horizon? What's the next frontier out there?"

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