US inflation data for August delivered a complex picture, with the annual core rate dropping to its lowest level in over five years while the monthly figure came in hotter than anticipated. This combination, set against a backdrop of resilient employment figures, has solidified market expectations for a September rate hike by the Federal Reserve.

According to data released by the Bureau of Labor Statistics on Friday, the headline Consumer Price Index (CPI) rose 3.4% year-on-year in August, matching both forecasts and the previous month's reading. On a monthly basis, the CPI increased 0.4%, a significant acceleration from the 0.1% gain recorded in July, and was in line with economist expectations.

When stripping out the volatile food and energy components, the core CPI advanced 0.3% month-over-month, surpassing the 0.2% that analysts had projected and marking the most substantial monthly increase since April. However, the annual core inflation rate eased slightly to 2.4% from 2.5% previously, representing the lowest annual figure since March 2021 and aligning with broader expectations.

Following the data release, US stock futures initially dipped before rebounding, with all three major indices trading over 1% higher as of this writing. Concurrently, gold prices climbed by 1.5%, while the yield on the 10-year Treasury note saw a modest decline.

Traders have now priced in an approximately 90% probability of a rate hike at the Fed's next meeting. In a preview analysis, JPMorgan's market intelligence team noted that a 0.3% monthly core CPI reading would necessitate a hike in September, whereas a 0.2% figure would have allowed the central bank to hold steady.

Why a 0.3% Monthly Core CPI Increase Raises Red Flags for the Fed?

A significant rebound in energy prices was the primary catalyst for August's overall CPI uptick. Energy costs surged 2.1% on a monthly basis, with gasoline prices jumping 3.9% and contributing to more than one-third of the total monthly increase in the headline index.

However, it is the core inflation data that truly influences the Fed's policy calculus. Housing costs rose 0.3% for the month and increased 2.75% year-over-year, continuing a downward trend in annual terms. Meanwhile, certain service-sector prices showed renewed strength, with communication services climbing 2.3% month-over-month, hotel accommodation costs rising 2.4%, and airline fares up 2.7%.

This data suggests that US inflation is not experiencing a broad-based reacceleration. Instead, it points to a state where the overarching downward trend persists while specific short-term prices are beginning to tick back up. For the Federal Reserve, it is precisely this latter dynamic that warrants close attention.

Market participants had previously identified the 0.2% versus 0.3% monthly core CPI reading as a critical threshold. The final print of 0.3% falls squarely on the side that is more likely to trigger policy concerns.

Was the September Rate Hike Sealed with August's CPI Report?

In the immediate aftermath of the CPI announcement, markets swiftly adjusted expectations for Federal Reserve policy. The probability of a 25-basis-point rate increase at the upcoming FOMC meeting surged to above 90%, nearing full pricing-in.

Brian Bingham, a short-term macro trader at Goldman Sachs, views the Fed as being in an extremely delicate position. He pointed out that a 0.3% core CPI reading is sufficient to support a rate move in September, even as some officials have cautioned against placing excessive weight on marginal differences in the data.