The local unions representing grocery store workers said Thursday that they’ve authorized a vote to strike after contract negotiations with Southern California grocery store chains faltered. Members of seven local grocery store unions will vote March 21 to March 23. The vote will determine whether a potential strike will commence. A strike in the region has not occurred since 2003. The United Food and Commercial Workers Union Local 770 said the contract for about 60,000 Southern California grocery store workers at more than 500 stores expired at midnight Sunday. After 12 days of negotiations w…