A Harrisonville teenager is dead after an accident Thursday afternoon in Cass County.  The Highway Patrol reports Savannah Nash, 16, failed to yield at the intersection of Highway 7 and Walker Road.  She was struck by a tractor trailer driven by Gary Moberly, 60, of Clinton.  Nash was pronounced dead at the scene.  Family members said Nash had just received her driver’s license.  Police said they found an unsent text message on the teen’s phone.