By Michael S. Derby

A new San Francisco Fed report casts doubt on the idea that the U.S. central bank has sustainably hit its inflation target, in a finding that could undermine the central bank's rate-rise plans.

The report says that while the outlook for price pressures is "broadly balanced," the author sees "a considerable possibility" that inflation will again fall short of the Federal Reserve's 2% inflation target.

The report, which was written by bank economist Adam Shapiro, observes that many of the forces that had been pushing inflation higher over recent months aren't tied to the overall performance of the economy.

This grab bag of so-called acyclical factors is set to wane, which means that inflation could again begin to ebb. "If non-health [services] acyclical inflation were to revert to more normal historical levels, it would imply some downside risk that future inflation would fall below target again," the report says.

The Fed's preferred price pressure gauge is the government's personal-consumption expenditures price index. Compared with the same month a year before, inflation has been at or above the Fed's 2% target for some months. In September, the overall index rose 2% from a year earlier, but it had risen 2.2% in August and 2.3% in July.

The San Francisco Fed report focuses on the core PCE price index, which strips out food and energy prices in a bid to get a better read on underlying price trends. That measure has held steady at a 2% 12-month rise since May.

If the paper's findings are right, inflation could upend the Fed's monetary policy outlook. The Fed has raised rates three times this year. Most officials and private forecasters expect another increase in December, from the current short-term target rate range of 2% to 2.25%. The Fed also has projected a series of rate rises into next year and beyond.

The reason for all those rate boosts? The Fed says it is trying to extend the economic expansion by tempering potential inflation gains via economy-slowing increases to short-term borrowing costs.

The main source of the upward pressure, as the Fed sees it, is the strong job market, with a 3.7% unemployment rate that is expected to move even lower. Historically, ultralow unemployment sparks inflation by pushing up wage pressures as firms compete for scarce workers.

So far, wage gains have been relatively muted. Also, inflation has for most of the expansion been too low, not too high. If the Fed is wrong about inflation rising, then it calls into question the need for higher interest rates.

San Francisco Fed President Mary Daly recently said some of the modest wage gains owe to the fact that new workers, who usually don't earn big wage gains, are being pulled back into the labor force by a strong economy. As these workers get seasoned and the economy keeps humming forward, wage gains could pick up.

"Every sign about the economy so far is pointing to additional rate increases being required, and December is another meeting that's on the table for that sort of decision," Ms. Daly said earlier this month. But she added, "I'm thinking seriously about that, but I don't want to be premature and say it's definite when we have a lot of time between now and December to see how the economy unfolds."

Philadelphia Fed leader Patrick Harker, however, increasingly sees a tepid inflation outlook as a reason for central bank caution.

"At this point, I'm not convinced a December rate move is the right move, " Mr. Harker said in an interview with The Wall Street Journal this month. "We're not seeing the recent data telling us that inflation's moving rapidly past our target. So I think we have some time to let this evolve."

Write to Michael S. Derby at michael.derby@wsj.com