Sales at convenience stores run by Japan’s 12 major operators fell 1.6 percent in May from a year earlier to 537.02 billion yen for the second straight monthly decrease, the Japan Franchise Association said Tuesday.

The percentage change is adjusted for variations in the number of stores.

On an unadjusted basis, sales posted a 3.9 percent gain. Association officials blamed the decline mainly on bad weather, which often fell on weekends during the month. They said this helped bring the number of shoppers down 0.3 percent and sales per shopper down 2.4 percent.