(Reuters) - U.S. auto makers are expected to report big declines in August domestic sales despite stepped-up incentives, the Wall Street Journal said citing a report by J.D. Power & Associates. Industry-wide sales, however, will improve slightly from July, it said. The research firm expects sales in the closing days of the month to improve, helped by General Motors Corp's 100th anniversary sales event, which offers "employee pricing" on most vehicles and cash rebates on selected light trucks. The J.D. Power report estimates sales of about 1.21 million vehicles for the month, or 13.4 million vehicles on a seasonally adjusted annual rate. The seasonally adjusted annual rate, or SAAR, is a closely tracked indicator of auto industry demand. U.S. auto sales had plunged to a 16-year-low in July, led by a 27 percent drop at General Motors, as high gas prices and tight credit sent the industry into a tailspin. "However, with auto makers getting near the end of one model year and getting ready to start the next, sales are beginning to show some signs of improving over July's dismal sales performance," J.D. Power Chief Economist Bob Schnorbus said. (Reporting by Sweta Singh in Bangalore; editing by Rory Channing)