SAN FRANCISCO/WASHINGTON (Reuters) – Federal Reserve Chair Janet Yellen signaled that the U.S. central bank will likely start raising borrowing costs later this year, even before inflation and wages have returned to health, but emphasized the return to normal interest rates will be gradual. A downturn in core inflation or wage growth could force the Fed to delay the first increase to borrowing costs since 2006, the central bank’s chief said on Friday, but policymakers should not wait for inflation to near the Fed’s 2-percent goal before tightening monetary policy.