Yichiro Chino/Getty ImagesAfter the August pullback, investors should prepare for the historically bumpy month of September.The market hasn't hit the bottom yet, Jeffrey Buchbinder of LPL Financial believes.Buchbinder shares 3 signs of a market turnaround and how to invest in the meantime.After a rocky start to August, the S&P 500 has all but clawed back its losses.However, LPL Financial's Chief Equity Strategist Jeffrey Buchbinder warns that more bumpiness could lie ahead as summer comes to an end.Buchbinder and his team pointed out in a recent note that September is historically the worst month for the S&P 500.