September Stock Market Outlook: Rate Hikes Loom Over Weakest Month

The stock market is heading into fall 2026 with a challenging combination of factors. The S&P 500 is just 1.3% below its all-time high, yet prices now suggest a 60% probability of a September rate hike.

Historically, September has been equities' weakest month, averaging -0.3% for the S&P 500 since 1950. This year's macro backdrop amplifies the pattern, with sticky 3.7% PCE inflation and overbought monthly technicals making the seasonal headwind sharper than usual.

The Federal Reserve's hawkish stance is a significant variable. Chair Warsh explicitly warned that summer PCE and CPI readings show no meaningful improvement in underlying inflation, and rate hikes 'may be needed.' A rate hike would push discount rates higher, capping multiple expansion and valuations compressing further.