The best-performing September in 15 years just sent U.S. stock indexes to new highs, but that party could soon end, with Goldman Sachs expecting a surge in market volatility as October kicks off a gauntlet of earnings and macro risk.

With just days left in the month, the S&P 500, tracked by the Vanguard S&P 500 ETF (NYSE:VOO), is up 3.6%, on pace for its best September since 2010. That year, the index jumped 8.76%, while in 1997 it rose 6.22%, two of the only times the market defied September's historically weak trend to this degree.

Tech stocks are doing even better. The Nasdaq 100, via the Invesco QQQ Trust (NASDAQ:QQQ), is up 5.5%, and the Technology Select Sector SPDR Fund (NYSE:XLK) has surged 7.5%, its second-strongest September since the ETF launched in 1999.

That's not normal. Over the last 25 years, tech stocks have averaged a 2.2% loss in September.

But this year, names like Oracle Corp. (NYSE:ORCL), Tesla Inc. (NASDAQ:TSLA), Micron Technology Inc. (NASDAQ:MU) and Apple Inc. (NASDAQ:AAPL) have led a momentum surge fueled by AI demand and ...