When water infiltrates into a snowpack, it alters the thermal state of the system. This work presents a first-of-its-kind multi-phase heat transfer model for local thermal non-equilibrium scenarios of water infiltration into an existing snowpack, such as during rain-on-snow events. The model can be used to calculate the formation of ice layers, as well as partial melting of the snow. Hence, it can support hazard assessment for flash floods and snow avalanches.