The stock market's breezy summer is over as investors brace for a bumpier fall. In recent months, equity investors cheered soaring profits at big companies and nudged megacap tech shares back near records. Now, as the post-Labor Day stretch begins, new challenges lie ahead: shifting odds of an interest-rate increase from the Federal Reserve, sky-high expectations after a stunning earnings season, and persistent inflation threats from Middle East tensions.

"You're moving from this earnings-driven market to this macro-driven market with the Fed, inflation and interest rates in focus," said Keith Lerner, chief investment adviser for Truist Advisory Services. "It tends to be a choppier period."

Historically, major U.S. stock indexes experience their worst average returns in September. The Dow Jones Industrial Average has slid an average 1.1% in September, dating back to the 19th century. The S&P 500 has seen the same average decline, and ends the month lower more than half the time for every September since 1928.

Analysts caution against reading too much into seasonal patterns. However, new reasons for investor caution have emerged, including the looming threat of an interest-rate increase from the Federal Reserve, which announces its next policy decision on Sept. 16. Chairman Kevin Warsh's decision to abandon forward guidance and take cues from markets has muddied monetary policy expectations, leaving traders scouring Fed speeches and economic data for clues.