(These are the market notes on today's action by Mike Santoli, CNBC's Senior Markets Commentator. See today's video update from Mike above.) A bit of high-altitude turbulence strikes, which probably feels worse than it is to many traders because the ascent has been so smooth for so long. At the headline-index level, a modest blip, the S & P 500 down half a percent at the worst, though in part the slide is being buffered by uncharacteristic strength in some defensive groups. The makings for some type of shakeout have been accumulating, noted here in recent days: Unusually low correlations among stocks and sectors, uncommonly extreme single-stock volatility relative to calm index action, remarkably heavy call-option volumes and some fundamentally flimsy speculative groups ( quantum computing

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