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Why I'm staying invested in overvalued market: Expert

August is a traditionally tough month for stocks, but for now investment pro Jack Ablin is staying invested in the market despite feeling that it is as much as 25 percent overvalued.

"Valuation is not by itself a timing tool so we can get a market that's expensive and keeps getting expensive," the chief investment officer of BMO Private Bank said in an interview with CNBC's "Power Lunch" on Monday.

"Generally what we like to do is stay with momentum, stay with a good thing, as long as it keeps going."

Trader on the floor of the New York Stock Exchange.
Brendan McDermid | Reuters
Trader on the floor of the New York Stock Exchange.

Ablin said that 25 percent overvaluation can also be cut a little slack because he looks at prices relative to sales, and profits are expanding.

"I don't view it necessarily as a threat. It can be a threat, maybe a 10 to 15 percent threat, but it is in no uncertain terms a headwind. There's no doubt that that's what's really preventing the market from going higher this year."

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However, Ablin said he's also becoming cautious as the internals of the market are weakening and momentum is starting to erode.

Another thing to consider is market sentiment, said Jeff Hussey, global chief investment officer of Russell Investments.

"U.S. markets are a little overpriced but sentiment is not that frothy," he told "Power Lunch."

"That gives us some headroom for prices to move forward."

Plus, once the Federal Reserve begins to raise rates, it will do so gradually and therefore "the cycle will remain fairly positive for equities to continue to do well," Hussey added.

However, he said he's a little more cautious on the United States than Europe, where prices are cheaper and sentiment is worse.

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—CNBC's Jennet Chin contributed to this report.