Economic data, earnings and political uncertainty could all inject a fresh bout of volatility into a market that has continued to broaden

September is ending, and it has been a mostly triumphant month for the stock market despite a rough start. But history suggests investors could wake up to more volatility in October.

To be sure, major indexes like the S&P 500 SPX are poised to buck a well-established historical trend by finishing September firmly in the green. With a gain of 1.6% for the month so far, the index is on track to clinch its strongest September showing in more than a decade when Wall Street closes on Monday, according to Dow Jones Market Data.

For nearly a century, the month has been the worst of the calendar year for stocks, and returns in recent years have been particularly disheartening.

A similar seasonal analysis suggests things could get rocky in October, too.

During election years, the stock market's September weakness has tended to spill over into October, according to data from CFRA's Sam Stovall that dates back to the 1940s.

But it's not just seasonal trends giving investors pause: Any one of a number of catalysts could arrive to rattle investors' confidence during the month ahead. Notably, while the Fed's jumbo interest-rate cut helped boost stocks in September, investors won't be able to fall back on the steady hand of Fed Chair Jerome Powell; the Fed's next policy meeting isn't until November.

In the interim, they will have to contend with a parade of economic data, with each new data point potentially unearthing new risks.

Even formerly second-tier reports like the Fed's Beige Book and retail-sales readings could swing markets as investors try to ascertain how much the labor market is slowing and where the economy might be headed next, said Jake Jolly, head of investment analysis at BNY Investments, in an interview with MarketWatch on Friday.

See: Why stock-market investors are freaking out over economic data they used to ignore

The first marquee report is due on Friday, when the Labor Department releases its monthly nonfarm-payrolls report for September. But investors will receive a flurry of other market-moving data on the labor market before then.

"That's the next big test for the market," Jolly said.

The start of the third-quarter corporate earnings season could also present challenges. After a strong second quarter, Wall Street expects America's biggest companies to report even more aggressive profit growth in 2025.

Terry Sandven, chief equity strategist at U.S. Bank Wealth Management, told MarketWatch that any guidance that challenges these expectations could deliver another setback to a rally that has seen the S&P 500 climb 60% from its October 2022 lows, according to FactSet data.

All the while, geopolitical and political risks lurk in the background. An "October surprise" that shifts the calculus surrounding the November election could leave investors rattled. So could signs that the fledgling conflict between Israel and the Lebanese militant group Hezbollah may be poised to escalate.

With equity valuations on the high side relative to history, any disappointment could quickly snowball, Sandven told MarketWatch.

"Stocks are priced for perfection right now, so there's a lot that needs to go right," he said.

A September to remember

Even if more volatility lies ahead, investors have plenty of reason to be optimistic about the market's long-term prospects.

For one, the number of stocks boosting the S&P 500 continued to expand in September, after an initial uptick earlier in the third quarter.

Investors have also proven eager to swoop in and buy the dip in stocks, suggesting that they still see value amid the tumult.

In September, as in August, the market kicked off the month with a nosedive, only to quickly win back its losses. According to Dow Jones data, this year marked the first time ever that the S&P 500 has finished higher for two months in a row following a drop of 4% or more during their opening days.

Ari Wald, chief technical strategist at Oppenheimer, told MarketWatch that the improving breadth, in particular, is a sign that the market remains on "healthy" footing.

"We're seeing these pockets of the market that haven't really participated to the same degree catch up," Wald said. "We think it's still the middle innings of a secular bull market."

This sector rotation, combined with the Fed's shift toward a policy of easing and the likelihood that strong earnings will continue during the third quarter, could "set up the stock market well going into October and the last quarter of 2024," said Jay Woods, chief global strategist at Freedom Capital Markets.

That said, some doubt that the market's gains from here on out will be as rapid as what investors have seen over the past two years.

After a long stretch of calm, volatility returned to markets over the summer. BNY's Jolly said that given the uncertainty surrounding how quickly the Fed might move to cut rates in the future, it's likely that investors will continue to encounter bouts of volatility on a more regular basis.

BNY expects the pace of the rally could slow in 2025.

With the two-year anniversary of the start of the bull market looming in October, investors could be tempted to look back and take stock of how far things have come. That could leave some with vertigo.

Stocks finished mixed on Friday, with the S&P 500 missing out on a fresh record high. But the Dow Jones Industrial Average DJIA managed to clinch a fresh record, while the Nasdaq Composite COMP fell, according to FactSet data.