U.S. auto retail sales are expected to fall in December, as supply shortages and high demand have caused prices to skyrocket, consultants J.D. Power and LMC Automotive said in a report released on Thursday. Retail sales of new vehicles could fall 17.7 percent to 2,923,600 units from a year earlier, the report said. “Intense demand with this limited supply is resulting in prices continuing to increase,” said Thomas King, president of the data and analytics division at J.D. Powers. Auto manufacturers of all sizes are grappling with the strongest inflationary pressure in three decades, following a relentless rise in raw materials prices. Average transaction prices are expected to reach $45,743, going above the $45,000 mark for the first time, and 20 percent higher than December 2020 when prices first breached the $38,000 level. Production at auto plants rose 2.2 percent last month after advancing 10.1 percent in October. But November …