Just before daybreak, mission managers called off the afternoon liftoff and said they would try again Friday.
Rain was pounding the area, and meteorologists said there was little chance the weather would break in time for Thursday's planned launch. The official forecast was 80 percent "no go."
On Friday, the weather outlook improves dramatically. There was a 60 percent chance that conditions will be acceptable for launch, although wind will be a concern once the front passes through.
Managers will meet again early Friday to evaluate the weather. If they feel they have a decent shot, they will give the go-ahead to fuel Discovery for liftoff. Liftoff on Friday would be at 3:03 p.m.
Discovery already has been delayed by gas leaks and an electrical problem.
The mission to the International Space Station is now running four days late. It will be the final flight for Discovery, which faces a museum retirement as the shuttle program winds down.
Six veteran astronauts are assigned to the 11-day flight. They have been at Kennedy Space Center for the past week, waiting out all the delays.
NASA has until Sunday to launch Discovery, otherwise the shuttle will remain grounded until the beginning of December because of unacceptable solar angles.
It's officially NASA's next-to-last shuttle flight. Endeavour is scheduled to lift off at the end of February. An extra mission may be added in mid-2011, if money is forthcoming.
The White House wants NASA focused on next-generation rockets and spacecraft that could carry astronauts to asteroids and Mars. The plan, for now, also calls for private business to develop rockets capable of carrying astronauts to the space station. Until then, American space travelers will need to hitch rides on Russian Soyuz spacecraft.
NASA: " target="_blank"> http://www.nasa.gov/mission_pages/shuttle/main/index.html