Investors will parse Yellen's language to determine whether the Fed will begin unwinding its balance sheet in September. The central bank previously said that was a move that it would implement "relatively soon," provided the economy expands as expected. Paring the Fed's sizable assets would likely tighten monetary conditions in the same way an interest-rate hike would. Another rate increase isn't expected until December at the earliest.
The Dow Jones Industrial Average fell 0.4%, the S&P 500 declined 0.34%, and the Nasdaq slid 0.3%.
Watch: 3 Things to Watch in Janet Yellen and Mario Draghi's Jackson Hole Speeches
Uber said it posted a loss of $645 million in the second quarter, narrower than the $708 million loss in the first quarter and the $991 million loss in the fourth quarter of fiscal 2016. Revenue was $1.75 billion, up from $1.5 billion in the first quarter and about $800 million during the same period a year earlier. Gross bookings, or total money collected by fares, jumped to $8.7 billion.
Uber has $6.6 billion cash on hand, compared to about $7.2 billion in the first quarter.
The company is continuing its search for a new CEO and Meg Whitman, chief executive of Hewlett Packard Enterprise Co. (HPE) , repeated that she won't take the top spot at Uber.
Sears Holdings Corp. (SHLD) reported a second-quarter adjusted loss of $1.16 a share vs. a year-earlier loss of $2.03. Sales were $4.37 billion compared with $5.66 billion in the same period last year.
Sears said Thursday it was still exploring opportunities for its Kenmore and DieHard brands. It also said it would close 28 Kmart stores later in 2017.
Tiffany & Co. (TIF) reported second-quarter earnings of 92 cents a share vs. 84 cents a year earlier as sales rose to $959.7 million from $931.6 million.
Earnings reports are also expected Thursday from Abercrombie & Fitch Co. (ANF) , Broadcom Ltd. (AVGO) , Staples Inc. (SPLS) , Perry Ellis International Inc. (PERY) , GameStop Corp. (GME) , JM Smucker Co. (SJM) , VMware Inc. (VMW) , Marvell Technology Group Ltd. (MRVL) and Brocade Communications Systems Inc. (BRCD) .
The consortium also includes private-equity firm KKR & Co. (KKR) as well as state-backed Innovation Network Corp. of Japan and Development Bank of Japan, the sources told Reuters.
Toshiba, the Japanese conglomerate, has been trying to sell its its memory chip business to cover losses from its U.S. nuclear business.