We consider a continuum model for the flow of two phases of different mobility and density in a Hele-Shaw cell or a porous medium. As a consequence of the Saffman-Taylor instability, the phase distribution is thought to develop a microstructure, so that its evolution is effectively unpredictable. We identify the constraints on the macroscopic quantities, like the averaged volume fraction of the phases, and show that these constraints allow to derive some predictions on how the macroscopic quantities change over time. Furthermore, we investigate a class of closure hypothesis, which complement these constraints and thereby determine an evolution of the macroscopic quantities themselves, by analyzing the stability of this evolution.