WASHINGTON (Reuters) - U.S. shoppers turned out in force in March, pushing retail sales to their strongest gain in a year, according to a government report on Tuesday that may prompt economists to raise forecasts for first-quarter growth. "The implications of these data are massive. Off the top of my head, I would say that we just moved from 4 percent GDP growth in Q1 to something like 5 percent," said Steven Stanley, chief economist at RBS Greenwich Capital. The economy expanded at a 4.1 percent pace in the final three months of 2003.