This paper presents an accurate physical model for the threshold switching effect in a Niobium oxide-based memristor. The proposed model takes inspiration from a mathematical description for the device behaviour, recently derived by the application of a nonlinear identification procedure to the differential algebraic equation set of Chua's Unfolding Principle. The model accurately captures the device nonlinear dynamics in both pre-And post-Threshold switching operation regions under distinct ambient temperatures. In the course of the threshold switching process the device internal temperature, set as the memristor state, undergoes critical changes which activate electronic conduction, but are insufficient to determine an insulator-To-metal transition. As a result the turn-on process is determined by electron flow only.