Atlantis is scheduled to blast off Monday at 2:28 p.m. on a mission to stockpile the International Space Station with big spare parts.
There's now a 70 percent chance that the weather will cooperate, which is not nearly as good as previous days. Low clouds are the main concern.
Atlantis' six astronauts woke up around the time fueling got under way Monday well before dawn.
The 11-day flight is expected to keep the astronauts in orbit through Thanksgiving. They plan to unload nearly 30,000 pounds (13,600 kilograms) of pumps, tanks and other spare parts, as well as science experiments.
It's NASA's last shuttle flight of the year.