The Bank of Canada on Tuesday sharply lowered its growth forecast for the third quarter and said the crucial export sector might accelerate slower than initially estimated. Senior Deputy Governor Tiff Macklem said the central bank expected annualized growth in the third and fourth quarters to be in the 2 to 2.5 percent range before strengthening in 2014. In its monetary policy report released in July, the bank said third-quarter growth would be 3.8 percent and fourth-quarter growth would be 2.5 percent.