Federal Reserve officials have become more pessimistic in their economic outlook through next year and have lowered their forecast for growth.

The economy will grow only 2.4 percent to 2.5 percent this year, Fed officials said Tuesday in an updated forecast. That's down sharply from a previous projection of 3 percent to 3.5 percent. Next year, the economy will expand by 3 percent to 3.6 percent, the Fed said, also much lower than its June forecast.

Fed officials project that unemployment won't change much this year, averaging between 9.5 percent and 9.7 percent. The current unemployment rate is 9.6 percent. Progress in reducing unemployment has been "disappointingly slow," the central bank said, according to the minutes of its Nov. 2-3 meeting.

The darker view helps explain why the Fed decided at its meeting earlier this month to launch another round of stimulus. The central bank plans to buy $600 billion in Treasury bonds over the next eight months in an effort to lower interest rates and spur more spending.

The Fed is slightly more optimistic about 2012, in part because officials expect the bond-buying program to have a positive impact. The economy should grow 3.6 percent to 4.5 percent that year, a tick better than June's forecast of 3.5 percent to 4.5 percent.

The economy will also grow 3.5 percent to 4.6 percent in 2013, the central bank said, the first time it has issued projections for that year.