U.S. stocks moved higher in quiet trading Wednesday as investors look ahead to a closely-watched speech by Federal Reserve Chair Janet Yellen on Friday. 'I think markets are looking for a bit of clarity from Yellen but I don't think they're going to get clarity around a September [rate hike],' said Simon Smith, chief economist at FxPro, based in London. Tesla (TSLA) - Get Report shares rose after unveiling updated versions of its Model S and Model X vehicles, which include a stronger battery that's able to propel the Model S from zero to 60 miles an hour in 2.5 seconds. Express (EXPR) - Get Report shares slumped 20 percent after reporting lackluster quarterly results. Earnings of $0.13 a share missed estimates of $0.17. Comparable store sales dipped 8 percent year-over-year, while analysts had expected a 4.6 percent drop. TheStreet's Scott Gamm reports from Wall Street.