Stocks open higher. A Yellen bounce?

NEW YORK (CNNMoney)

Stocks powered ahead at Wednesday's open ahead of key testimony from the Fed chief on the state of the U.S. economy. Her testimony begins at 10 a.m. ET.

Traders are also buzzing about Alibaba's long awaited U.S. initial public offering, which could be the biggest in U.S. history.

The Dow Jones industrial average opened up over 50 points, and the S&P 500 has solid gains. The Nasdaq is lagging behind, falling into negative territory as investors continue to mull the health of the tech sector.

Related: Five things to know about Alibaba

Janet Yellen's script today is likely to mirror the Fed's statement last week when it decided to dial back its bond-buying program by an additional $10 billion. But she has surprised markets before.

"The market is positioning itself for (more) dovish comments [from Yellen]. That means that we would get a bigger reaction in both bond and [currency] markets to relatively hawkish comments, but it is hard to imagine those happening," said Societe Generale strategist Kit Juckes.

Marc Chandler, global head of currency strategy at Brown Brothers Harriman, said, "Her comments today are not expected to deviate much in substance from the FOMC's statement last month."

Related: It's the Janet Yellen show

In corporate news, Whole Foods (WFM, Fortune 500) shares plummeted 19% after posting earnings that fell short of expectations and dimming its guidance.

Likewise, AOL (AOL) retreated 11% as an ad-driven rise in revenue was overshadowed by tumbling profits.

Shares of Groupon (GRPN) dove 14% after the daily deals company logged a larger-than-expected quarterly loss.

Related: Fear & Greed Index still gripped by fear.

Brewer Molson Coor (TAP)reported that "underlying after-tax income" more than doubled during the quarter, compared to one year ago, but net sales slipped and worldwide beer volume was flat.

Shares of Anheuser-Busch InBev (BUD) flatlined after the brewer reported lower-than-expected profits. Tesla (TSLA) and SolarCity (SCTY) are set to report after the close.

Despite holding a major stake in Alibaba, Yahoo (YHOO, Fortune 500) shares fell over 4% in early trading following the big IPO filing announcement.

U.S. stocks closed in the red on Tuesday. The Dow Jones industrial average lost nearly 130 points and the Nasdaq fell 1.4%. The S&P 500 fell 0.9% after advancing on each previous Tuesday this year.

Related: CNNMoney's Tech30

European markets were mostly moving up in morning trading.