There may be some cloudy days ahead for the stock market after the strong summer run. The S & P 500 is on pace to end August 2% higher after rising 2.2% in July. The benchmark is also on pace for its fourth straight monthly advance. But the index typically averages a 0.7% decline in September, making it the worst month of the year historically, according to the Stock Trader’s Almanac. With the S & P 500’s July and August rallies, September could bring even rougher waters based on historical patterns. Looking only at years when the broad index gained more than 2% in both July and August, that average September slide increases to 1%. Of those 11 years going back to 1949, all but three have ended in the red. Additionally, seven of those 11 years had a negative October. The average loss for the month was 2.9%. “With the history of volatility toward the end of Q3 and into October there is the potential for some late summer/early fall selloff,” Jeffrey Hirsch, author of the Almanac, said in an email. Hirsch said third-quarter weakness could come later than it has in previous postelection years. That would make sense given that first-quarter market difficulties were delayed this year, he said. .SPX 3M mountain S & P 500, 3-month chart Regardless, Hirsch said to view any pullback as short-lived. He reaffirmed his best-case scenario for the market this year, which would have full-year gains of between 12% and 20%.