Earlier this month I featured a chart of the S&P 500 and the 50-day moving average. We saw that benchmark indicator provide classic support for our favorite index.

Today the S&P 500 dropped decisively through the 50-day MA. Over the next few days market technicians will be watching the follow-on behavior. Will the selloff accelerate? Or will today's 2% decline prove but a brief dip below this classic indicator? We'll post the occasional Stockcharts.com snapshot as this "encounter" plays out.

Update: The selloff today was on increased volume — the highest since the September monthly close.