Stormy weather could stall this weekend's launch of space shuttle Endeavour.

NASA will try to launch Endeavour on Saturday night, following a pair of delays last month caused by a hydrogen gas leak. The leak has since been plugged. But now, NASA will have to deal with thunderstorms.

Forecasters on Wednesday put the odds of acceptable weather at just 40 percent for a Saturday evening launch. Conditions improve only slightly on Sunday.

The seven astronauts assigned to the 16-day mission are back at the Florida launch site. They will deliver and install the final piece of Japan's lab at the international space station.

NASA's countdown clocks will begin ticking late Wednesday.