The last space shuttle launch this week may have to deal with bad weather.
Nasa forecasters said there was a 60% chance that rain or thunderstorms will delay Friday's planned lift-off of Atlantis.
The weather improves with each passing day, as the launch time gets slightly earlier.
Friday's launch time is 11.26am (1626 BST).
The four astronauts assigned to the 12-day flight arrived at Kennedy Space Centre on Monday.
Atlantis will make one final supply run to the International Space Station before retiring. As many as a million spectators are expected to jam Florida's Cape Canaveral area for the historic send-off.