By John S. McClenahen With the U.S. economy likely to remain weak at least until midyear, it may seem premature to be anticipating higher short-term interest rates. But that's exactly what the economists at Global Insight Inc., Waltham, Mass., are doing. They expect the U.S. economy will be growing at a 4% annual rate during the second half of 2003 and figure that the 12-member Federal Open Market Committee, which sets monetary policy, will boost the target for the influential federal funds rate to 1.75% at its Dec. 9 meeting. The funds target currently is 1.25%, a 40-year low. By the summer of next year, the forecasting firm's economists predict, the federal funds target will be 2.5%.