An important property in a sensor network is the monitoring of temporal changes of hazardous situations such as forest fires. Rescue groups need to be aware of dynamic changes that affect their rescue efforts. In this paper, we discuss a sensor network model that provides a good abstraction of geometric and topological features of a dynamically changing sensing environment. This model enables efficient path planning and navigation using localized algorithms. We propose a dynamic medial axis model that represents shapes and changes of shapes in a geometric space. We develop distributed algorithms to capture the dynamic network topologies. Dynamic medial axis allows rescue teams to find a short path to safety in a changing environment. We show that our dynamic medial axis algorithms provide good approximations to the true medial axis and our routing scheme generates short and safe routes. The simulation results show that the routes found by our scheme are near-optimal.