Morgan Stanley senior economist Vincent Reinhart is predicting that the rebounding economy will sputter again in the first of this year and that the Federal Reserve will need to jump start it with another round of "quantitative easing."

In a research note that was published on the web site Business Insider Thursday, Reinhart attributed the recent improvement in the U.S. economy to two key factors: falling energy prices and "the unwind of negative shocks" from Japan's devastating earthquake and tsunami. "Once these tailwinds have played out," he wrote, growth will slow and the Fed will need to step in and buy up between $500 billion and $750 billion of Treasury and mortgage-backed securities.

The Fed has twice embarked on large-scale asset purchases since the start of the financial crisis, most recently in November 2010. In his note, Reinhart predicted that while internal disputes within the Federal Open Market Committee will delay action for a time, the Fed will take action some time between March and June.

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