EAST TEXAS — A powerful cold front has sent temperatures plunging across Texas as far south as the Rio Grande Valley, where the National Weather Service has issued a freeze watch through Tuesday morning. The freeze watch for far South Texas could endanger crops and other sensitive vegetation. Much of the state Sunday was under some form of winter weather, freeze or wind chill advisory, watch or warning. Texas Department of Transportation crews last week pre-treated 10,000 miles of roads with a brine solution to keep ice from sticking. Department spokeswoman Natalie Galindo says crews in the Fort Worth and Dallas area have been spreading a magnesium chloride granular mixture on roads to help drivers. Icy conditions have been reported north and west of Dallas through the Panhandle and West Texas.