US stocks slumped more than 1% on Thursday ahead of Federal Reserve Chair Jerome Powell's speech at Jackson Hole, Wyoming, on Friday. The S&P 500 lost 59.7 points, or 1.35%, at 4,376.31. The Dow Jones Industrial Average fell 373.56 points, or 1.08%, at 34,099.42. The Nasdaq Composite dropped 257.06 points, or 1.87%, at 13,463.97. Central bankers have gathered on Thursday for an annual symposium at Jackson Hole. Shares of Nvidia rose as much as 6.7% on Thursday, hitting an all-time high, after the company unveiled a $25 billion stock buyback plan and record quarterly revenue. During the session, the company…