Sudden squalls almost on top of the launch pad early this morning forced postponement for at least a day of the flight of the space shuttle Discovery, aborting the liftoff five minutes before the five-man crew was to depart on a daring salvage mission.

"The mission has been scrubbed," mission control told Discovery Commander Joe H. Engle and his crew at 9:06 a.m. EDT.

Minutes later, the sky along the shuttle's flight path was bright blue.

If the ever-changing weather of central Florida cooperates, the launch of the 20th space shuttle mission will take place on Sunday between 7:57 a.m. and 8:11 a.m. EDT. That 14-minute period is the day's "window," during which Discovery would have to blast off in order to accomplish its assigned tasks on an eight-day mission. Among the factors determining this flight's launch window is the position of the satellite to be salvaged and the time that three communication satellites must be deployed.

One of the satellites Discovery will carry into orbit is owned by the American Satellite Co., another by the Australian government and the third by the Hughes Communications Corp., which plans to lease it to the Navy.

Once the crew deploys its satellite cargo, it will try to revive a Navy communications satellite that has been dead in orbit since April. Astronauts James D. van Hoften and William F. Fisher are scheduled to don space suits, go outside the cargo bay and attempt to "hot wire" the lifeless satellite much as a thief might start a car by bypassing its ignition.

Discovery had been scheduled to lift off today at 8:38 a.m., but a series of unforecast thunderstorms blew in from the mainland directly across the flight path. Dark thunderheads surrounded the launch pad for five miles in almost every direction. Lightning and rain could be seen in the distance.

National Aeronautics and Space Administration officials worried not only about the shuttle climbing through rain and lightning but about the crew's ability to see the Kennedy Space Center runway in case they had to make an emergency landing in the early minutes of flight.

Also, rain could damage the shuttle's fragile tiles, and lightning could harm its computers and guidance systems.

There is no guarantee that Discovery will go off on time on Sunday. The kind of weather that overtook the central Florida coast this morning is forecast again. If anything, Air Force weather officers are expecting more Atlantic squalls on Sunday than occurred today.

If Sunday's liftoff is scrubbed, the crew -- Engle, van Hoften, Fisher, Richard O. Covey and John M. Lounge -- will try again on Monday. There will be launch opportunities on Tuesday and Wednesday as well.