Friday is supposed to be the last space shuttle liftoff for NASA, but forecasters say there's a good chance that rain or thunderstorms will prevent Atlantis from flying.
No hail or other severe conditions are anticipated, and mission managers say the weather would have to get worse before the launch is called off.
A rain-free, storm-free zone is needed 23 miles around for the shuttle to take off.
Up to 1 million people are expected to jam the area for the launch of the final mission. And the weather appears to be the only uncertainty.
NASA says if Atlantis isn't flying by Sunday, it'll have to wait until at least July 16.