(Adds policy context, updates market reaction)
NEW YORK, Sept 5 (Reuters) - The Federal Reserve should delay raising U.S. interest rates until it is confident inflation that is now "well short" of target will rebound, an influential policymaker said on Tuesday in the clearest signal yet the Fed is getting more dovish in the face of weak data.
In a speech that zeroed in on months of sliding price readings, which depressed a key measure to 1.4 percent, Fed Governor Lael Brainard said the U.S. central bank should go so far as to make clear it is comfortable pushing prices modestly above a 2 percent target.
The comments from the dovish official, combined with those of her colleagues, suggest that months of falling or flat inflation readings could scuttle the Fed's plans to raise rates once more this year and three times next year. They may be the strongest hint of caution at the Fed ahead of a Sept. 19-20 policy meeting.
"We should be cautious about tightening policy further until we are confident inflation is on track to achieve our target," said Brainard, a permanent voter on monetary policy who in the past has convinced colleagues to delay tightening.
"I believe it is important to be clear that we would be comfortable with inflation moving modestly above our target for a time," Brainard said at the Economic Club of New York.
The Fed has raised rates twice this year including in June, when it published forecasts. Investors have since grown skeptical and now give a December rate hike a 27 percent probability, down from 30 percent before Brainard spoke. Her comments also helped depress yields in U.S. bond markets.
Brainard drew a similar line in the sand a year ago, helping delay a policy tightening by a few months. Two weeks ago, Fed Chair Janet Yellen's other sitting governor, Jerome Powell, said low inflation allowed the Fed to be patient on a hike.
Brainard noted the U.S. economy is on "solid footing" and benefiting from two years of an "extremely welcome" rebound in its global peers. She also suggested she supports announcing a reduction of the Fed's $4.5 trillion balance sheet this month, as widely expected.
Yet the policymaker seized on the fact that core U.S. price readings have sagged for five straight years. Such depressed underlying inflation, combined with a lack of dangerous financial asset bubbles that would force the Fed to raise rates more quickly, made the case for caution, she said.
It also appeared the Fed would need to drive unemployment, already low at 4.4 percent, "considerably" below an equilibrium level to help boost inflation, Brainard said. Only a quick rebound in prices would allow the Fed to follow its June forecast, and even then it could reach an effective rates ceiling "without too many additional rate increases," she added.
Addressing Hurricane Harvey last week, Brainard said it raises uncertainties for the U.S. economy and will likely have a "notable" effect on growth in the third quarter, though that should be followed by a year-end rebound.
She added that she has revised her predictions for fiscal stimulus from the Trump administration and U.S. Congress, given that early-year expectations of tax cuts and infrastructure spending have been downgraded. (Reporting by Jonathan Spicer and Stephanie Kelly; Editing by Chizu Nomiyama and Meredith Mazzilli)