The Federal Reserve's September meeting is the most important one in recent memory, amid expectations that central bankers will not raise rates, but announce a start date for the trimming of its $4.5 trillion balance sheet.

That's the assessment from George Rusnak, co-head of global fixed income strategy at Wells Fargo Investment Institute. 

Rusnak doesn't expect any balance sheet changes to spark overreactions in the stock or bond markets. He said investors seem ready for such changes. 

While Rusnak expects a pause in rate hikes at September's meeting, which ends Wednesday, he thinks the Fed will raise rates in December, sticking to their initial 2017 forecast of three rate hikes. 

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