WASHINGTON — An index designed to predict the future health of the economy posted a second straight strong increase in May, indicating the economy should gain strength in the second half of this year.

The Conference Board says that its index of leading indicators rose 0.7 percent in May, matching the gain in April. Both months represented the strongest increase since a 1 percent rise last July.

Conference Board economists say that the two months of strong gains support the expectations that the economy will keep expanding in the final six months of the year.