Thailand's annual inflation rate was negative for a second straight month in May, the commerce ministry said on Friday as it cut its full-year forecast to close to zero, just weeks ahead of the central bank's next monetary policy review. The consumer price index dropped 0.57% in May from a year earlier, not quite as steep as the 0.80% fall forecast in a Reuters poll, following a 0.22% fall in April. It was the third month in a row that the inflation rate has been below the central bank's target range of 1.0% to 3.0%.