Renowned stock analyst Ralph Acampora, director of technical research at Altaira Investment Solutions, has a bleak outlook for the market.

While he forecasts a 10 to 15 percent decline for the S&P 500 by October, he sees a steeper drop for small-cap, mid-cap and technology stocks.

The Russell 2000 index of small-cap stocks, the S&P MidCap 400 index and the tech-heavy Nasdaq Composite index are headed down 20 to 25 percent, he tells CNBC.

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"I call it a stealth bear market going on."

The S&P 500 slid 0.94 percent Thursday, and the Nasdaq Composite lost 0.76 percent. The Russell 2000 has fallen 9.6 percent from its March 4 high.

Many momentum stocks stood at overvalued levels early this year, Acampora explains.

"A melt-up actually began in December of last year and carried into the first quarter of this year. A lot of the favorite stocks, like Facebook and Amazon and all those, were way ahead of themselves," he notes. "I don't think we're even close to unwinding that excess.

"The last time I saw anything like this [the way the markets are behaving] was in 1994, when the Dow and the S&P were in a 10 percent trading range all year, and then under the surface they were just ripping them apart. I have a sick feeling that we might be doing that again."

To be sure, Acampora maintains stocks "will be very strong" in the fourth quarter.

Many experts expect a continuation of the rotation out of small-cap and growth stocks. Stocks are "starting to form a little bit of a ceiling," Uri Landesman, president of Platinum Partners in New York, tells Reuters.

"The small caps have really been selling off, and I think you're continuing to see a rotation out of anything the market considers riskier."

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