Update (12:12 ET): 'Walloped!' is the descriptor that CNN/Money is now leading with: 

The Dow Jones industrial average dropped 420 points, or 3.7%, after falling as much as 528 points. The S&P 500  was down 46 points, or 3.8%; and the Nasdaq Composite  lost 106 points, or 4.3%.

Update (10:40 ET): The Dow continues downward, falling 500 points.

On the heels of the Labor Department's rising jobless claims report and Morgan Stanley cutting its global growth forecast, U.S. markets opened weakly on Thursday morning with the S&P 500 and Nasdaq falling 3 percent. Via Reuters:

The Dow Jones industrial average was down 317.00 points, or 2.78 percent, at 11,093.21. The Standard & Poor's 500 Index was down 37.87 points, or 3.17 percent, at 1,156.02. The Nasdaq Composite Index was down 92.33 points, or 3.68 percent, at 2,419.15.

Morgan Stanley's pessimistic growth forecast, in particular, cast a cloud over the morning trading: "The fact that Morgan Stanley has downgraded its global growth forecast really highlights the concerns and problems facing the global economy," said market analyst Michael Hewson to CNN/Money. "It begs investors to question where future growth will come from."