A key U.S. forecasting gauge rose in August for the fourth straight month, suggesting economic growth will strengthen further in the second half of the year, a private research firm said Thursday.

The Conference Board (search) said the index of leading indicators (search) rose 0.4 percent in August, matching expectations of Wall Street economists, after an upwardly revised 0.6 percent gain in July.

"The economy is improving, although the road will likely remain bumpy," Conference Board economist Ken Goldstein said in a statement. "With export growth still months away, the growth burden remains on consumer spending and business investment."

The Conference Board said the 2.5 percent increase in the leading index since its low in March has already been followed by stronger real gross domestic product growth in the second quarter.

"In addition, the recent strength in the leading index suggests a further strengthening of economic growth in the second half of the year," the board said.

The leading indicators index is now 1.4 percent higher than the previous peak reached in May 2002.

The Conference Board said the coincident index was unchanged in August after a 0.2 percent rise in July. The lagging index, a measure of past trends in the economy, was unchanged after a 0.1 percent increase in July.

It said four of the 10 indicators that make up the leading index rose in August. The positive contributors were interest rate spread, vendor performance, real money supply and building permits.