The trend is even clearer in two alternative measures that Ellis Tallman, director of research at the Cleveland Fed, likes to track: median CPI and his bank's “trimmed mean” CPI. Instead of taking out the segments that are considered historically volatile, a trimmed mean removes whichever components moved the most in that period, and so reveals something closer to the central trend.

“We don't see inflation rearing its ugly head,” said Tallman. “On the other hand, we do see inflation in the medium term heading back toward 2 percent.”

Still, against the sharp drop in unemployment over the past six years, coupled with the Fed's inaction on rates, that's a puzzlingly slow rise.

One explanation: ”Wages have not grown really as strongly as they should have, despite the fact they didn't move interest rates,” said Jeremie Banet, a fund manager at Pacific Investment Management Co. in Newport Beach, California.

Productivity the Culprit?

Banet and his colleagues have dug into CPI figures looking for a purer signal on what's happening with wages. They narrowed their focus not only to services, but also tossed out rents and their equivalents, as well as energy. The resulting “core services ex-shelter” is up more than 3 percent in the year through August, significantly higher than average hourly earnings reported by the Bureau of Labor Statistics.

To Banet, this evidence that prices are rising faster than wages is a clue. Companies raise wages when productivity is high. Right now productivity is low. “That could explain why wages are not rising as much as you'd expect,” he said.

If true, and productivity remains weak, that would also support Yellen's decision to remain patient, as wages would be less likely to contribute meaningfully to an upward spiral in inflation.

Fuel Injection on the Way

Less divination is required when it comes to the impact of fuel prices on headline CPI. Since mid-2014, oil prices have plunged, with one of several big dips coming in late 2015 into this year. The impact of that drop is about to roll out of the year-on-year figures for the energy component of CPI.

In the 12 months through August, motor fuel prices dropped nearly 18 percent. Should fuel prices remain exactly where they were at the end of August, the year-on-year deflation will disappear by November and turn into inflation that reaches 22 percent for fuel prices by February. Given the 3.26 percent weighting for motor fuels in overall CPI, that would result in an upward swing of more than 1.3 percentage points in CPI from August to February.

The caveat here is that Fed officials regularly ignore swings in headline inflation caused by energy and food price changes. Janet Yellen has long spoken of the “transitory” nature of oil's impact on inflation. But with a historical bias guarding against high inflation, can the Fed ignore energy on the way up, as easily as they do on the way down?

“I think they can,” said Gennadiy Goldberg, an interest-rate strategist at TD Securities LLC in New York. “But if there is a little more inflation on the core side at the same time that headline is rising, that could catch their eye.”

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