LOS ANGELES—Grocery store workers and Southern California supermarkets have let their labor contract expire on the morning of March 7, and a strike could be in the future. Contract negotiations between the United Food and Commercial Workers Union Local (UFCW) 770 and Southern California’s major supermarkets broke down over the weekend. The existing contract expired at midnight. Supermarket workers picketed stores in Baldwin Hills and Tustin last week while the union and supermarkets were in negotiations. The union posted on Twitter to its members that despite not reaching an agreement, the terms and conditions of the previous contract are still in effect. “When we began negotiations with Ralphs and Albertsons/Vons/Pavilions in January, we came prepared with comprehensive proposals and a clear path toward negotiating a contract that reflects your value and the sacrifices you have made,” the union said in a series of tweets to its members. “We are disheartened …