  We propose an importance sampling scheme to estimate the partition function
of the two-dimensional ferromagnetic Ising model and the two-dimensional
ferromagnetic $q$-state Potts model, both in the presence of an external
magnetic field. The proposed scheme operates in the dual Forney factor graph
and is capable of efficiently computing an estimate of the partition function
under a wide range of model parameters. In particular, we consider models that
are in a strong external magnetic field.
