U.S. stock futures and oil prices were tanking again Thursday morning, and closely followed market watcher Dennis Gartman said he believes the selling is going to persist.

"I don't think there's too much selling at all. I think there's going to be even more selling going on," the founder and publisher of "The Gartman Letter" told CNBC's "Worldwide Exchange" Thursday. "Sovereign funds are in the process of liquidating. They have no choice."

In early trading, Dow futures were off more than 300 points at one stage, U.S. crude was falling more than 3 percent to under $27 per barrel, the 10-year Treasury yield was dipping below 1.6 percent, and the dollar was down about 1 percent against the Japanese yen.

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"I'm afraid it's going to get even worse. I hate to say that. There's not a good tenor to be found anywhere," Gartman said.

Against this backdrop, Federal Reserve Chair Janet Yellen goes back to Capitol Hill Thursday morning to testify on the economy and monetary policy on the Senate side.

Investors will look to see if she changes her tune from Wednesday's approach before a House panel, which combined a steady-as-she-goes account on interest rate policy with an acknowledgement of intensifying risks from markets and the slowdown in China.

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But Gartman said Thursday: "The central banks I think at this point are as confused as is anyone else. I'm not sure anyone is going to look to central banks to be of great help right now."

The spiral downward in the markets is "margin liquidation," he said, with no indication when the selling might end.

"It will end when it ends," he said. "I've lived through many of these in 40 years of being around [markets]. Suddenly there's just an end to the selling, everyone looks around and realizes that the selling reached a tsunami-like ending. And you turn out."