(Reuters) - The U.S. economy is a "fiscal train wreck" waiting to happen that risks ushering in a period of stagnation featuring by minimal growth, high unemployment and deflationary pressure, U.S. economist Nouriel Roubini wrote on Friday.

In a commentary for the Financial Times, Roubini -- one of the first economists to predict the housing crash in the United States and known as 'Dr Doom' for his pessimistic forecasts -- said fiscal and monetary stimulus had prevented another depression.

But he said that further quantitative easing likely to be announced by the Federal Reserve next Wednesday will have little effect on U.S. growth in 2011, "so fiscal policy should be doing some of the lifting to prevent a double dip recession," he said.

He said the U.S. remains on an "unsustainable fiscal course" and the likely make-up of Congress after elections next Tuesday, in which the Republicans look set for strong gains, virtually takes fiscal reform off the agenda.

"The risk ... is that something on the fiscal side will snap ... The trigger could be a debt rollover crisis in a major U.S. state government," he wrote.

"The worst of the coming fiscal train wreck will be prevented by the Fed's easing. But the risk is (Obama) ... will then preside over ... a Japanese style stagnation, where growth is barely positive, and deflationary pressures and high unemployment linger."

(Reporting by John Stonestreet; editing by Patrick Graham