(Reuters) - The Federal Reserve on Tuesday began a two-day meeting against the backdrop of a weakening U.S. economy that will likely force policymakers to plan for the possibility that things may get worse.They'll have to acknowledge that the recovery has decelerated, but want to avoid fanning the fires of QE3 expectations," said Ward McCarthy, chief financial economist at Jefferies & Co, referring to a potential third round of bond buying, or quantitative easing