Stocks Good Until Summer, Then Watch Out: Expert

The stock market should continue higher through the end of the second quarter, former UBS Americas Chairman Robert Wolf told CNBC on Wednesday. But he said he's "nervous" about the second half of the year.

Wolf, CEO of consultancy 32 Advisors, also said in a "Squawk Box" interview that the Federal Reserve should start tapering its easing monetary policy "sooner rather than later."

(Read More: Taper by 'Fat, Dumb and Happy' Fed Will Hurt: Pro)

He agreed with hedge fund titan David Tepper, who told the program on Tuesday that scaling back the Fed's bond-buying program is needed to keep stocks going up at an even, measured pace.

The founder of Appaloosa, Tepper also said he's still bullish on stocks, which helped propel the Dow Jones Industrial Average to a record 18th-straight Tuesday increase. The S&P 500 Index also closed at all-time high.

(Read More: It's a 'My Cousin Vinny' Market, Bullish Tepper Says)

Fueling the recent rally, Wolf noted, are inflows into stock mutual funds and the Fed forcing investors into the risk trade.

"The stock market will continue to run at least for the second quarter, because GDP in the second quarter will be very good," Wolf said. "I think you could see 3 percent [GDP]. More to the ups than the downs."

He noted that housing, manufacturing, retail sales and employment are all showing some signs of strength.

"The second half of the year, I'm a little more nervous, because usually with a good stock market you see good M&A activity," Wolf said. "We've seen very little M&A activity, and not a big pipeline. Organic growth is [also] a little bit slow."

By CNBC's Matthew J. Belvedere. Follow him on Twitter @Matt_SquawkCNBC.