The Minnesota Nurses Association abruptly told hospital negotiators Thursday that it intends to file a strike notice today, setting the stage for a walkout against Twin Cities hospitals that could begin as soon as July 5. The surprise notification came after negotiators for the two sides sat down with a federal mediator for the first contract talks in almost three weeks. The nurses voted overwhelmingly on Monday to authorize a strike, but union officials said they would postpone calling a strike as long as there were "productive" negotiations.