Warning that things could change again Monday, the National Aeronautics and Space Administration today predicts that Skylab would fall from orbit and to break up over Angola and South Africa on Wednesday morning.

Revising its earlier estimate that Skylab would break up Wednesday afternoon over the United States in a diagonal line from the state of Washington to Florida, the space agency today placed its bet that Skylab will reenter the atmosphere over the Cape Verde Island off the west coast of Africa, descend through the atmosphere in a southeasterly direction and start to come apart as it passes over Angola.

Should the prediction prove correct, the first 200 to 300 of an expected 500 pieces of derbis would probably fall in Angola and South Africa as Skylab's hulk moves from southern Affrica toward the indian Ocean. The largest and heaviest pieces would drop short of Australia into the Indian Ocean, endangering little more than its large population of whales.

According to today's NASA forecast, the moment of random reentry is Wednesday at 10:28 a.m. EDT. That is what the space agency now considers the most likely time that Skylab will start its fall to Earth and scatter 26 tons of derbis along a 4,000-mile path.

Over and over, NASA emphasizes that its predictions are subject to errors of as much as 24 hours. Skylab could fall as early as 7:28 p.m. EDT Tuesday or as late as 1:28 a.m. EDT Thursday. It is even possible that Skylab could survive in orbit until the end of the week.

"We're talking about a very imprecise set of projections," says NASA's deputy associate administrator, Richard G. Smith. "There's always a chance that solar activity will increase, expand the atmosphere and put more drag on the vehicle, and bring it down sooner. There's just as much chance that the top of the atmosphere might shrink and delay the end."

When Skylab plunges, an estimated two-thirds of the 77 1/2 tons in orbit will burn up in the atmosphere. The 500 pieces that fall will range in weight from 3 pounds to 5,100 pounds. The lightest pieces will fall first, since they won't have much forward velocity. The heaviest pieces fall last, near the end of a 4,000-mile track that NASA calls the Skylab footprint."

Ten pieces will weight more than 1,000 pounds each, which means they'll strike Earth at speeds between 150 and 260 miles an hour The lightest of these is a telescope spar that weights 1,100 pounds. A 1,600-pounds apiece -- are expected to survive reentry. A lead-lined film vault that weighs 3,9000 pounds and an airlock shroud weighing 5,100 pounds are expected to be the last to hit.

NASA insists that all the debris will fall along a path no more than 100 miles wide, that no more than three pieces will fall in the same 100 square miles, and that the chance of any person being hit by any of the pieces is 152 to 1.

"You're talking about a particle distribution of at the most maybe one every 10 miles," Smith said. "You're not talking about a rain of debris."

If Skylab fall as NASA today predicted, the safest place for the inhabitants of Africa would be a basement or a tall building, but not the top tow floors. Smith believes the chances of being hit are so remote that the most dangerous action would be to race to a place considered safe.

"If it was me, I'd go about my business," Smith said. "If you go in the basement, obviously you're a little better off, but I think we're talking about such a minimal risk that I personally wouldn't do anything. I would not change my lifestyle."

The question Smith hears most are why can't the Air Force shoot Skylab down with a missile and why can't Russian cosmonauts fly to Skylab and boost it into a higher orbit.

The answers are simple. Blowing up Skylab with a missile, presuming a missile could be put into orbit to intercept Skylab, would produce a large number of more dangerous pieces that would fall to Earth. If Russian cosmonauts could attach their spacecraft to Skylab, which they cannot do, they would endanger themselves by firing their engines to carry it to a higher orbit. The jolt might pull of one of Skylab's solar panels, which could tear a hole in a spacecraft. CAPTION: Picture, no caption