Crotty market update: The S&P 500 surged through August, but will a double-top spell double-trouble come fall?

As we exit the Labor Day holiday after a summer bull run, we sit on the precipice of market-shaping events that could change the world as we know it. Here's what investors need to know.

August is historically a cruel month for investors, but this August was not. Though volatile, the S&P 500 benchmark index posted a 2.3% gain, with the Dow climbing 1.8% for the month and the Nasdaq Composite Index up 0.7%. It was not spectacular, but it was reassuring. August marked the fourth straight month of gains for the S&P. The year-to-date re…