U.S. stocks have sharply outperformed international stocks so far this year. 

Such a divergence is unusual in a bull market and U.S. stocks may start to cool off and reflect the global growth worries. 

That's the assessment from David Keller, President & Chief Strategist, Sierra Alpha Research, who thinks a retest of the 2018 lows of 2,581 in the S&P 500 are unlikely. 

The S&P 500 is up 8.6% since the start of the year.