As emergency crews fortified sandbag barriers and shoveled away muck in flood-stricken California, President Clinton appeared live on TV to assure victims that federal help was on the way. At least six deaths have been blamed on storms along the West Coast. By last night, Clinton had certified 24 California counties north of San Francisco and near Los Angeles as disaster areas. He will visit the scene in the politically important state early next week, on a trip originally planned to mark the first anniversary of the Northridge earthquake. While storms abated throughout the state today, forecasters predicted more rain tonight in Southern California and said two other Pacific storms would hit the state by the weekend.